I don't trust my manual targeting.
If I have a good solution I try one torp in the center to get the best chance of a hit.
If I have a bad solution I put one aft and one on the bow for the best hit chance.

If you don't watch them, and leave the area, will the sinking to confirm later? Or is it true that you must shadow and witness the sinking to take credit?

As long as you remain within rendering range (about 30km) you should be okay.

I tried Danlisa's bow shot yesterday on a Small Merchant nw of the Isle of Man -- and it worked just fine. The ship began to draw water and in a couple of minutes the propeller was spinning out of the water. She sank in somewhat more than four minutes.
Will try with bigger ships, if I cross some before the Brits sink me...

Gute Jagd

Buhring

P. S. June 1941, 90% realism (external views, used only for eye candy), GWX 2.0