Its quite true that the actual range at which the shot was taken is problematic
The exact details of this shot have always been rather sketchy. The distances involved, either of Plunkett's advance or of his shot, and Plunkett's motivations are vague. The main problem is with the unreliability or absence of eyewitness accounts. Three Riflemen left accounts, Captain Kincaid, Quarter Master Surtees and Rifleman Costello, yet none of these men were present and base their accounts upon regimental legend. Of those who were in the retreat, Lieutenant Smith and Rifleman Harris both leave comprehensive memoirs that give no mention of Plunkett. Harris, excusably, as he was with the 2nd/95th on the road to Vigo. Similarly, William Napier does not mention him in his history of the war, despite his family's heavy involvement in the Light Division. Basically, the only information available, although from apparently primary sources, is from secondary accounts and hearsay. Some secondary research by the re-enactment society for the 95th suggest they have a fair idea of where the shot was taken, and it does seem to be less than the 600 yds claimed 9certainly less than the 800 yds some of the more outlandish claims) but also certainly considerable more than 300 yds French sources often try and claim.
The main area of argument is the range at which Plunkett made the shot. Oman, in stating that it was 'from a range that seemed extraordinary to the riflemen of that day', but giving neither an exact figure, nor any sources, seems to have started to establish the myth of Plunkett making an impossible shot, a myth frequently repeated by modern authors as proof of the prowess of the Baker Rifle and those who used them. Some popular literature even puts the range at 800 yards as suggested above. Other accounts are less emphatic about the range. Kincaid records only that Plunkett took up an 'advanced position', and Costello that he ran 'about a hundred yards nearer to the enemy'. As no record is left of how far the distance between the lines was at that moment, this does not help much. The most intriguing account is Surtees'. He says that Plunkett 'got sufficiently nigh to make sure of his mark', insinuating that the range was quite normal.
All of these seem to put the emphasis on Plunkett's bravery in advancing so far forward as to make sure of his shot, rather than the range at which the shot was made. The debate probably began much later than the time of the diarists' writings, possibly brought about by the complete lack of evidence on this point. Even Rutherford-Moore, after close analysis of both sources and the ground at Cacabellos concludes that the range could have been 'anywhere between 200 and 600 metres'. This covers the difficulties in balancing the various factors involved in the shot. Plunkett would need to be at a close enough range to hit a moving target despite his breathless and frozen state, yet still be able to beat the speeding cavalry to make good his escape. Another factor needs consideration if the story of his then downing a second Frenchman is to be believed
Based on reloading times, the need to be able to get back to friendly lines ahead of the french cavalry, and the claim that plunkett brought down a second frenchman, the best estimate is that the shot was probably taken at somewhere in the vicinity of 450m (my estimate)