Well said von zelda!
Doing that you minimize the distance the torp has to run to meet the target + you also minimize the risk of torp not detonating due to bad angle when it meets the target.
An impact at 90 degrees is the optimal. Off course if the target is going slow it doesn't make much of a difference, but it does if it's going fast (aka warship). That's the way i do it also by now but i tried to give an "as simple as it gets" version
