Great explanation Joe!
There is another very simple no calculation way of doing this:
ID and range find your target
Select the torpedo depth, speed and pistol settings before raising the scope for the shot.
Open all tubes
Raise the scope and range find again. I like to be within 800m if possible.
Wait for the target's bow and the first mast to cross your reticle. Fire one.
Wait ten seconds. Fire two.
Wait ten seconds. Fire three.
Wait ten seconds. Fire four.
If the target is moving too fast for a 30 second spread adjust your reticle towards the direction of travel between shots.
If it's too small for this many torps just fire two or three rather than all four.