Gamble's recent book on Rabaul gave some good info on No.75 RAAF squadron's performance and made the purchase worth it for that alone. There's some wiggle room but the optimal estimate is a 3:1 ratio in favor of the A6M ...
I expect to get Gamble's book for Christmas

, but on 75 Sdn at Moresby in Mar-April 1942, I previously compared Wilson "Seek and Strike" (history of 75 Sdn), with the original combat reports of 4th and Tainan Air Groups, the Zero units present. I checked Gamble's sources on Google books and it seemed he used Wilson and Japanese Monograph 120, which is less detailed. The ratio you quoted from his book would seem to include Zero losses which were not to Kittyhawks. I found 2 Zeroes actually downed/crashlanded due to Kittyhawks in this period:
April 5: this report is actually missing, but several other sources agree PO2C Yoshie Takuro was KIA in combat with Kittyhawks this day
April 17: PO3C Goto Ryosuke*, forcelanded, WIA, after combat with K'hawks
Other Zero losses:
March 22: PO3C Kikuchi Keiji KIA, defensive fire of RAAF Hudsons
March 23: PO2C Yoshii Kyoichi KIA, AA, both sides' accounts agree
April 7: PO2C Tan Yukihisa KIA, defensive fire of USAAF A-24's
April 17: PO2C Sakai Yoshimi, 'suicide crash' in the combat report, but a Japanese first hand account says he dropped from formation and crashed into a mountainside of the Owen Stanley's on the way *to* Moresby, perhaps oxygen system failure.
April 28: PO3C Maeda Yoshimitsu hit a tree on a strafing pass near Moresby, crashlanded, POW (per Allied accounts), a strange incident since Maeda took off on an interception of Hudsons over Lae, was simply missing from Japanese perspective.
April 28: (then) PO2C Nishizawa Hiroyoshi (eventually the leading JNAF ace) ditched on return from a cover flight for bombers, but it reported no contact with allied a/c.
75 Sdn lost, per individual accounts in Wilson, 16 Kittyhawks to Zeroes, though one of those losses, April 17, has no corresponding Japanese claim. So it was more like 8:1 in favor of the Zeroes in that first deployment, though the Kitthawks did also shoot up some Zeroes on the ground at Lae.
The RAAF Kittyhawks did better however in the few engagements in August 1942 while based at Milne Bay:
Aug 11: PO3C Yonekawa Shokichi ditched, WIA, 75 and 76 sdn lost 2 Kittyhawks and pilots each; 75 Sdn claimed 2 Zeroes, AA claimed 3 and 3 probable.
Aug 23: a/c of PO2C Yamazaki Ichirobei heavily damaged, counted as 'expended', Wilson does not give any claim by Kittyhawks though 1 was engaged.
Aug 27: Lt. Yamashita Joji, PO1C Yamashita Sadao, PO2C Kakimoto Enji, and Sea1C Ninomiya Kihachi all missing. Allied accounts make clear that Kakimoto (POW) was downed by AA, the other three probably by Kittyhawks though one might possibly have been to B-26's. One Kittyhawk was lost.
So probably 4 or 5 Zeroes downed v 5 Kittyhawks in this period. Or, it would come out ~3:1 in favor of Zero if you count both the earlier and later deployments together.
*note: most of these names appear somewhere or other in English language sources with these transliterations, but a few don't, and in those cases I'm using what I think is the most likely pronunciation; even native Japanese speakers can't tell always tell a name's pronunciation for sure just from seeing it written in Chinese characters, as in these reports.
Joe