Thanks for your replies, cpnscurvy. Is there anything you don't know?
I have experienced some of the difficulties connected with manual targeting myself, but it doesn't ruin my game experience at all. Many factors made it hard to set up a precise firing solution in real life, and I therefore accept the in-game difficulties as well. After a little playing you somehow get a feeling if your solution is good or not. If I got a bad feeling about my solution, I just launch a torpedo salvo with wider spread angles than I normally would, like 3-4 degrees between each torpedo. Most of the time I get atleast one hit this way (unless the "lucky" torpedo turns out to be a dud, in which case you can hardly blame my firing solution).