The torp you saw was probably a circle runner. These were torp failures causing the torp to circle back on you and run in circles until they died out. Many subs were lost due to this failure.
There are many different shooting methods. Check out this thread, skippers bag of tricks. It gives you about every possible shooting method, most with videos. Not sure what you mean by eyeballing it. There are simple methods such as the OKane 90 attack you would like and complex shooting methods such as 8010 for the insane WW1 skipper....... Using the stad is really very simple, so just watch a tutorial on manual targeting. Post in the thread, RR will make a shooter out of you.
http://www.subsim.com/radioroom/show...146795&page=11
Playing stock I could easily get 100K per patrol. With realistic mods I have patrols with a few ships, 20K ton range, good patrols of 100K tons. If I cut cams and contacts off and play 100% it goes down. Tonnage means nothing. If you play and ignore anything except capital ships, harbor rape you can stack a high score.