There's actually a training pamphlet from WW2 on the internet somewhere that shows the rudiments of firing from a gunners position in a bomber. I'll have to look for it again. I clearly recall it saying your aim point for a diving attacker on the bomber's six o'clock was a point in front of the attacker some number of times the exposed tail-to-prop distance of the attacker. So if it was dead level six o'clock, the exposed distance was zero, so you aimed dead on. If it was coming in at an angle and the exposure was 10 feet tail to prop, you aimed 10 feet (or 20 or 30, can't remember) in front of the prop. Or something like that. If I find it I'll post a link.