I do all the plotting in my head as a true Kaleun! I begun with 90° attacks and watching how torp goes with F12. That way now I do most of my shots with my eye. More or less speed and distance (under 1000 its just an eye, above little help with "double" view to determine dist in thousands), aob is simple, when going from ~90° its easy, when inside the convoy and knowing their course i know the exact aob and do crazy "arc" shots (torp goes stright, than turns to hit it's target from 90°
In SH3 ships don't do crazy things like advanced zig-zag mentioned in Das Boot book so it's very very simple with some experience. The key is not to use the map (it all started from real navigation), just rely on your course, add or subtract values, you'll know exactly what is happening and how in your imagination. When you hear a contact reports from your SO you know by your instincts where is the enemy. Just like an experienced Kaleun did.
PS I've tried slide ruler - it's amazing but didn't have time to do the maths when in the heart of convoy.
PS2 I use only impact triggers with results like huge tanker wiped with one eel...
good luck