Forget the stadimeter. Try a Fast-90 shot. With this you don't have to worry about range, only speed and AOB. It sounds like you've lined yourself up perpendicular already so most of the work is done for you (hint when the ship passes in front of you its AOB will be 90 degrees).
Rockin Robbins has a nice tutorial around here somewhere that describes this method very well (Dick O'Kane), its the only method I use actually, its very easy and works well even on long shots.
edit: here it is:
http://www.subsim.com/radioroom/show...6&postcount=67