Touche on your comment RR! Yes you can not have a god mode and stop time. Completely unrealistic. I put tons of hours in on SH3 GWX at perfecting my manual targeting skills. Observations, distance/speed conversions off observations, all that good stuff. I've got using my AOB calculator and making observations down to a science, mate. No alt-tab or pause needed.
Once I have course and speed entered into the computer AOB is easy and fast to come up with. My course + target course + bearing. Timing target zigs comes into play sometimes so AOB is not too difficult there either. My method is to observe general course, and get an angle on bow reading after the first zig, wait for the second zig, then fire on the third zig. Doesn't take enough time to pause game or alt-tab. I've got a pretty effective system set up. It's working for me, and that's really the key, right? Every captain has their own method, and what works for them, works.