As a general rule, I want to use one torpedo detonation per 6-8,000 tons.
But in order to get a torpedo detonation, I may have to launch more than one torpedo to compensate for the dud rate.
Then I have to factor in how many torpedoes I need to launch to get the right number of torpedo hits. Torpedoes do miss either due to my error (90%) or other causes (10%).
Depending on the circumstance and the firing solution, I may need to launch three torpedoes in order to make sure I get the intended one torpedo detonation. This, I feel, is a realistic strategy that I believe Kaluens had to deal with in real life.
Now if you are playing with no duds and auto targeting, this won't apply to your game play.