I think the way gunsights in WW2 worked, is you have a number of preset dials, each representing wingspan of different aircraft at certain distance. When engaging enemy, you select appropriet option, and light reflection will be put on your gunsight. So when enemy plane's wings are touching borders of the reticle, you know it's that far from your aircraft.
I know during last months of WW2, P51s were started to be equipped with some new type of gunsight, but can't recall what's all the deal about it was. Apparantly it made even more easier for someone to shoot enemy plane down.
mx22