I actually play at 95% realism: I keep the stabilized view. That makes it easier to use the stadimeter to estimate the range.
But honestly, I use the Watch Officer when I'm on the surface. I have him give me the range and bearing to the closest target (which is almost invariably the one I want to attack anyway).
One thing that will help you is to get Hitman's Kriegsmarine Whiz-wheel. The thread is here:
http://www.subsim.com/radioroom/showthread.php?t=114351
To do the targeting calculations, you just need the backside of it:
It works, it's somewhat authentic The frontside is completely authentic, Hitman copied it from a picture of a real one. He had to 'imagine' what this side looked like, but I think he did an excellent job.
Anyway, you can use one of those to find the AOB and speed of a target given two observations consisting of range and bearing, with any reasonable period of time separating the two.
The only real drawback is that the target has to have made an apparent movement of at least 6 degrees in bearing, so for very slow targets it takes a while to get the necessary movement.