The circumstances of the sinking were made in reports to the admiralty, with the numbers of bomb hits and/or near misses studiously recorded in those reports. We also know the numbers of IJN Vals committed to the attack, so combining those two sources of information it is not hard to verify that the hits to bombs dropped works out at an 80% hit rate.
WWII cruisers has a good summary of the damage suffered by the two cruisers:
"The Dorsetshire (Captain Agar) and Cornwall (Captain Mainwaring) had sailed from Colombo at 2200 4th April. During the night orders were received from the commander in chief to join him at 1600 5th April and at about 0700 course was altered for this rendezvous. About an hour later the report of strong enemy forces to the east of the cruisers was received in the Dorsetshire, and speed was increased to 27.5 knots, the maximum Cornwall was able to proceed, in order to join the commander-in-chief as soon as possible.
It was a calm day, with little or no cloud, and a slight haze over of the sun; visibility was extreme. At about 1100 a single aircraft was sighted by the Cornwall and reported to the Dorsetshire. It was a long way astern estimated at about 20 miles, and was lost sight of before it could be identified. Two other aircraft were sighted about 1300, one some 14 miles ahead, and judged to be friendly, and one astern, which closed to within 14 miles and was considered hostile. In view of the proximity of the rendezvous and the risk of the enemy aircraft detecting the commander-in-chief, it was decided to break wireless silence and make a shadowing report.
At about 1340, 5th April, three aircraft was sighted by the Dorsetshire overhead at high altitude: assuming them to be hostile, she opened fire immediately, but within a few seconds they dived on the Cornwall which was about a mile on the port quarter, and released their bombs at a low altitude, the first striking the after hangar and the second scoring a near-miss port side forward. Simultaneously, a separate formation of three aircraft attacked the Dorsetshire; avoiding action was taken to starboard, but all three bombs hit, one through the quarter deck disabling the steering gear, the second through the catapult, disabling both wireless offices, and the third port side amidships, putting out of action all the port anti-aircraft armament, except the pom-pom. Attacks by a succession of formations of three followed at intervals of a few seconds. Hits were received through the base of the foremost funnel, putting A boiler room out of action; on the quarterdeck, disabling X turret, and at the base of the after funnel bringing it down and at the same time blowing up the H.A. magazine. This bomb also put both pom-poms out of action.
Four minutes after the initial attack the ship took a definite list to port; all communication between the bridge and other parts of the ship had gone, and looking down from the bridge aft, all that could be seen was flames and smoke extending to the stern. At least four more hits were received making a total of 10, in addition to several near-misses, and two minutes later the captain realising the ship was sinking, ordered all hands on deck. This was followed in half a minute by the order to abandon ship, by which time she was slowly turning on to her beam-ends. Enemy aircraft were still flying low over the ship and using their machine guns, and it was possible to get a clear picture of them and their occupants. Almost immediately afterwards she capsized and sank stern first. It was incredible how quickly the ship sank - she just took one plunge by the stern, and as we were thrown into the water, the bows towered up almost vertically alongside us. All around was wreckage of sorts and oil fuel, though not very thick.
Meanwhile Cornwall, though still afloat, had not fared much better. For the next two minutes after the initial attack, bombs fell almost continuously, some scoring direct hits, but the majority were very near misses. Explosions followed each other so quickly that no exact idea could be formed of their sequence, or of the separate effect of each bomb. Altogether 8 hits and six near misses were sustained; only 2 complete misses were observed.
A near miss port side, abreast the Bridge, flooded large sections of the port bilges and dislocated electric power suppliers all over the ship. All personnel in the after engine room were killed (it is believed) by a near-miss on the starboard side, abreast the hangar, and both boiler rooms were flooded by near-misses and had to be evacuated. A bomb on the water line abreast the hangar, burst in the foreward engine room, which rapidly filled with steam and smoke and had to be evacuated, most of the personnel being saved; hits were also received between X and Y turrets, near the Dynamo room, in the sick bay flat and in the recreation space. Many of the Bridge personnel were killed or wounded by splinters of a bomb which exploded on the Starboard paravane; in one case at least the Bridge was also machine gunned. An oil bomb produced a flame which enveloped the starboard pom-pom and fore starboard superstructure, swept right over the air defence position and 8in director. Battledress and anti-flash gear saved all except the gun crew whose clothing caught fire. In less than five minutes after the attacks started all power had failed, and both boiler rooms and both engine rooms were flooding rapidly. The port gunwhale of the ship was awash, and the starboard outer propeller was breaking surface, the ship being slightly down by the bows. Thick black smoke was issuing from the foremost funnel uptake casings on both sides of the upper deck.
By 1351 the last bombs had been dropped, the Dorsetshire had sunk, and endeavours were being made in the Cornwall to correct the list to port and to get away a report by wireless. It soon became evident the ship was sinking, and the Captain, who continued to direct operations though wounded in the right shoulder, gave orders to prepare to abandon ship. He then with the remaining Bridge personnel, left the Bridge and making his way aft saw to the launching of all available floats. At 1355 the final order to abandon ship was given.
While this was being done, the Japanese aircraft, which had ceased attacking, either because it was obvious the ship was doomed, or more probably because they had expended all their bombs, formed up and flew over the survivors. 27 dive bombers and one floatplane were counted; there was no machine gunning of the men in the water.
The end was not long in coming. Some four minutes later the Cornwall heeled over to about 70 degrees and went down by the bows, taking her final plunge at an angle of about 30 degrees to the vertical and making singularly little disturbance. Once again the Japanese airmen had attacked and their dive bombing had been as successful as the torpedo and high-level bombing used against the Prince of Wales and Repulse. The attacks were carried out by single-engine 2 seater planes diving steeply at an angle of from 60 degrees to 80 degrees from ahead or fine on the bow. Their tactics were facilitated by the ships being on a southerly course, thus enabling them to combine the advantages of attacking down sun and from ahead - the cruisers blind spot for AA defences. The attacks were well pressed home, the height of release estimated at from 500 ft to 1000 ft and the bombing was remarkably accurate. The bombs were dropped in such a manner as to have one falling at the moment the previous one was exploding. The size of the bombs dropped was thought in the Cornwall to be between 250 and 500 pound. Some burst on impact and in some the explosion was delayed. The great effect of the near-misses which burst underwater was remarked on. So sudden and severe was the attack that the main HA armament of both cruisers was put out of action before more than a few rounds could be fired. The short range weapons continued to be fought right up to the last; the Cornwall is credited with having damaged one aircraft and probably destroyed another which was seen to hit the water.
A surprisingly large number from each cruiser survived these devastating attacks. They found themselves adrift in a shark-infested ocean, 300 miles from land, and with the scantiest of boat accommodation. Of the Dorsetshire there remained only 2 whalers - leaking badly - a skiff, a floatanet, 2 Carley floats, some rafts and odd bits of wreckage. The wounded were placed in the boats or rafts, and the remainder swam round collecting wreckage and joining it up as rafts around the boats. In a short time all survivors from the ship numbering over 500, formed one single body in the water, and were under perfect discipline and control. A couple of miles away Cornwall's survivors were doing much the same. In their case a whaler, a motor boat, 2 large oiling fenders and six floats formed the rallying point. Help could not be expected to reach them for 24 hours, but they were cheered in the late afternoon by the appearance of an Albacore, followed shortly by a Fulmar, which passed them a message to hold on.
About sunset an aircraft believed to have been hostile was sighted flying very high overhead. Then the night closed down. All that night and the following day they drifted in an oily calm, the wounded and exhausted in the boats, which were loaded almost gunwhale under, and the remainder in the water, holding on to what they could. The spirit, fortitude and discipline of the officers and men of both ships throughout this ordeal, as well as during the action, is described as beyond all praise. Soon after sunrise, 6 April, the heat became intense, and head coverings were fashioned from pieces cut from battle dress. Sharks were numerous; but these seemed content to wait for the corpses which were from time to time committed to the sea, and made no attempt to molest anyone living. Both parties were short of food and drink; what they had was sparingly doled out at intervals. In the late afternoon, 6th April, the appearance of an Albacore put good heart into everyone, and shortly afterwards the masts of a cruiser and two destroyers were sighted over the horizon to the south-west. These proved to be the Enterprise, Paladin and Panther, and their 30 hours ordeal was nearly at an end. 1122 men in the water, for the most part exhausted and with a large number of wounded, were brought on board within the space of an hour without the loss of a single life.
19 officers and 215 ratings were lost from Dorsetshire and 10 officers and 180 ratings from Cornwall".
From this source;
http://www.armouredcarriers.com/battle-for-ceylon-hms-indomitable-formidable/
"In total, the strike included 18 Vals from Soryu, 18 from Hiryu and 17 from Akagi.
By 1348, Dorsetshire's bow lifted as her stern began to slip beneath the waves. In all, she'd been hit by 17 500lb bombs
Japanese reports are cited as saying 13 bombs were dropped on Cornwall, with 11 direct hits. The heavy cruiser went under, bow first, at 1400
Egusa's dive-bomber crews had set a record for bombing accuracy: Every bomb either struck the heavy cruisers, or burst right alongside.
Spotted some two hours later by a Swordfish sent to investigate the scene, a rescue destroyer would be recalled by Somerville under the mistaken belief the Japanese main force was nearby".
The Japanese reports are more or less confirmed by the after action reports to the RN admiralty.
It should be noted that Akagi's Air Group did not actually participate in the sinkings. By the time this air group had arrived, the two cruisers were sinking already. Akagi's air group was several minutes behind the attck formation led by Lt Obusa and Tomonaga.
You can play games with these reports all that you like, but it appears that from the available source information that the IJN achieved 30 hits out of 36 attacks. That's a hit rate of 83%.