...but give it a try before giving up on it
Read the tutorial for it, it really helped me in general; even with fleet boats now, I think about manual targeting much differently and more accurately.
It was one of the many things that made my recent full 1939-1945 playthrough, so big thanks karamazovnew. I felt extremely proud of my best shot in the campaign - two torpedoes at two targets in column (so I only had to calculate the solution once), two hits, the longest after a running time of 7:43, i.e. distance of 7145.3m. MAN that felt good after entering all the firing data by hand! So the method works, and oh so much better than the default manual targeting. Hats off