If you are using OLC GUI mod (or if you not - try it out), there is a brilliant video tutorial for it available at this mod's post. Search mod forum for it, here you have OLC GUI for GWX 2.0 thread:
http://www.subsim.com/radioroom/showthread.php?t=124639
It's really a great video, covering all aspects of manual targetting with this mod, although the guy is definitely smoking too much
