Proviso: it all depends on my position and the situation. Is it an unescorted merchant in the middle of the North Sea or Atlantic -- or is it close to shore or in a convoy with the clock ticking on air/destroyer response? In the former, I can just freely reposition for a 90-degree shot but not so much in the latter.
Assuming my typical SNAFU approach angles

I like two-shot mag salvos with next to 0 spread against 7k+ ton targets, under the smokestack if I can to disable her. It's insurance against premature/dud torps and provides a very good likelihood of at least one under-keel hit. I don't use manual torp calculation. I'm proud of myself for just figuring out ruler/compass/protractor intercepts.
N.B.: In rough seas this isn't a great option because you never know at what actual depth the mag torps are going to run.