You'd need some basic calculus to determine the precise distance traveled based upon the initial speed, altitude and angle of release in conjunction with gravitational acceleration - a bit complicated although probably simpler than it would actually be in real life since I don't *believe* (I could be wrong) that air friction against the bomb is modeled in the game, which would introduce terminal velocity as a factor. ( I believe this because your bombs dropped from any altitude always seem to travel exactly the same distance as you do, such that you are always directly over the bomb strike when it strikes. In reality, they should fall farther behind as they lose the forward momentum.)
However, a quick and dirty calculation is easy - a very rudimentary approximation can be simply done from the pure dive angle using the formula minimum altitude = 1000*sin(dive angle) - especially as dive angles tend to be fairly steep. A more precise approximation would be to resolve the velocity at the time of release into it's forward and downward components, and use the classic distance/time/acceleration formula s = ut + 1/2*a*(t)2 to determine time to impact, and then solve for the the distance traveled along both vectors between the initial release and the impact.
Someone who still remembers how to do calculus would have to give you the precise formula to account for the parabolic path, however, if you wanted "exactly exact".
Given that you don't know precisely your angle and only your approximate speed and altitude and only the approximate altitude of the target I'd say that the quick and dirty simple formula - i.e. at a 45 angle a release at 707 feet would cause the bomb to travel the requisite 1000 ft in order to arm, and at a 60' angle, 866 feet is required, etc. is more than sufficient.
Although if you can judge your angle of approach, the altitude of your target, and keep track of your altitude precisely enough to actually take advantage of knowing that the minimum altitude to release in a 60 degree dive is 866 ft not 1000 (or about 0.4 seconds at 250 mph, and you're likely going faster) and still drop on the target and not hit the ground along with your bomb, you're better than me...
