Don't shoot 'till you see the whites of their eyes, men!
Average range for me is less than 1000m, can be troublesome when trying to manuver in the midst of a convoy, but usually assures a hit following the one ship one torpedo ethos
I find 500m is just about right... as for longest shot- 2000m t2 tanker. Maybe I'll have to practice my sniping towards the end of the war in the hopes of reducing detection.