there, see? Can of worms
seriously though, read up on the many many guides there are here, both on convoy attack and manual targeting.
90 degree is not always the best. early in the war it's asking for duds, and yes if you want to be at 90 on torpedo-firing-time, then you have to be at another angle when you start the run-up.
oh i could go on for hours and hours, but i seem to remember my most fun times with this game were learning it all by experience
edit: look into GWX and read the GWX manual. it's only about 400 pages