Rich, it's different for different planes.
With the F6F for instance, at ~ a 45 degree dive angle or greater and speed from a 10K dive you can pretty much put the pipper right on the target. It's part of why I always say the F6F is one of the best dive bombing platforms in the game.
Firing rockets from a 190A-8 from a lower alt with less dive angle, the target needs to actually pass out of sight under your nose to hit.
For every plane and for every circumstance (begining alt, speed, dive angle) it will be different.
As others have said, practice is your best bet.
BTW, you'll know you hit a hanger if you don't see any bomb craters.