I'll bet there are sub captains who never had a dud torpedo, or at least were not aware of it because they may have classified a dud as a miss in the heat of battle. Other captains may have had many duds.
The real point to me is that you can't create a mathematical formula within the game to represent a historical perception regarding dud torpedoes. We know there were duds, and we can estimate approximately how many.....but the key word is approximately.
If this is to be included in TM, please make it an Add On.