If you are truly interested in manual targeting, you need to make yourself a whiz-wheel (or as a second option, use a flash or in-game version).
Here is the thread that contains the Kriegsmarine wheel made by
Hitman:
http://www.subsim.com/radioroom/show...gsmarine+wheel
And here is a thread where you can grab an *EXCELLENT* set of instructions to use the wheel written by
klh:
http://www.subsim.com/radioroom/showthread.php?t=126824
If you couldn't be bothered to actually build one, you can use the flash version, but quite frankly I like actually using something physical.
Oh, if you want, you can also use a slide rule. I actually use a slide rule to do my AOB calculations when I use the Aspect Ratio method, it's more intuitive to me than doing it with the wheel.