Fishu, kill mark of 300 is easy to make, although it would work for quite a few pilots.
Besides, you would benefit the most out of this policy

Simply put the number "300" plus the marks for every other aircraft shot down after 300th, until the pilot reaches the next round number (400, for example).
Also, didn't real Luftwaffe paint Knight Crosses on the tails also ? How about that feature ? Cool indeed.
Allied planes had kill marks near the canopy, afaik. Make that too.
Still, reset the kill marks after the pilot is killed. If pilot bails or ditches, he is still alive, right ?
Doesn't it look cool ? Art guys, how about it ?