The torpedo angle is automatically calculated in the TDC unless you have that switched to manual. Either way, it may not always be the best. I find if it's a slow target I can calculate in a bit of time. Though one of my last patrols, I found nothing for well over 2 weeks. Any contact reports I got, were going the wrong direction, or too fast to intercept. I did manage to intercept a destroyer, though he was moving perpendicular and zig-zagging, too far away. I was on the surface at night. Fired my deck gun twice to get his attention, to no avail. So went to AA guns and fired at him. The tracers must have got his attention. Dove below thermal layer until he was at about 2000 yards, came up to periscope depth. He was zigging towards me, no good for a TDC shot. So I opened four forward tubes, and fired a spread visually when he was closing under 900 yards. Experience I guess because I score a hit and sank him.
P.S. Only a boring patrol would make me go after a lone DD like that, heh heh!
He was like 1600 tons or so.