You may apply whatever logic you want. Hartmann was almost exclusively, but not 100%, on the Russian Front, where confirmation of anything that went missing was not easy nor necessarily in any record.
I take the WWII totals as they were awarded during the conflict by the various air forces, except for ones that used a points award system.
You are free to take them as you want, but you make the classic statistical mistake of thinking that everyone's efforts will follow the Normal Distribution. That is rather emphatically not the case. You also have to remember, Hartman hunted alone a lot of the time, with only his wingman tagging along. The other top 2 weren't nearly as much of lone hunters as Hartmann was. I'm pretty sure that had a lot to do with it.
I have no way to verify Hartmann's victories, but if I were wondering about the top aces' victory lists, I'd look at Hartmann (352), Barkhorn (301), and Rall (275); aces 1, 2, and 3, and then look at, say, aces number ... say ... 25, 26, and 27, which would be Schack (174), Lang (173), and Schmidt (173) and see how their lists compared with primary sources. Of course, I don't HAVE and do not see how I can GET German primary sources. What percent "stack up" for these three?
If they came out significantly different from one another, then I'd say to look at something like the top 30 aces (all Luftwaffe) and see how much variation seemed normal. It would entail a significant study, which is something I'd actually DO if I could find the primary sources to do it with. To date, I cannot find those sources, and I am left with the scores as awarded. I will NOT arbitrarily change someone's score because his victory list seem to be abnormal to someone in a forum.
By the way, when you say 25% certification rate for Erich Hartmann, what sources are you comparing his kill list with that show a 25% verification rate?