As far as the F4U-1A, (and all the bubble-top Corsairs, for that matter) you generally want your target between the top of the cowl and the bottom of the gun sight. Exactly where depends on your speed, dive angle, and altitude of release.
Of course, if HTC would ever double the size of the F4U's sight, you could use the Mk.8 sight, which had 5mil hash lines below the outer ring just for that purpose...