I use auto-TDC, and with this and the right situation (no heavy fog and time to position the sub for a shot) I aim for the bow.
I'm just telling u because many Kaleuns here fire 2 torps at equal or bigger C2 ships for a sinking on the safe side when engaging convoys. But if u have the time to think about which torp to shoot when at what ship and at what speed, then u can as well take the time and make a good shot and sink 4 big ships with only four eels.
Of course there are situations, in which u don't have time and u just can react and shoot a magnetic.
@maillemaker: the picture above shows a sinking C3. These sometimes survive a bow shot and won't sink. Or maybe in this case i was to impatient to wait 30mins for her to sink

Here is another pic. These two T3's slow down and sank with 1 shot each, and the oil didn't catch fire or something.