Sorry about the confusion.
The torpedo depth gauge in game is imperial measurements. The paper manual however is in metrics. For example the Yamato (in the manual) shows a depth of 10.9 meters. To convert that depth to feet you multiply 10.9 x 3.25 which gives you a depth to keel of roughly 35.5 feet. You would then sub tract 12 feet from that (to count for the running too deep error) and you get 23 feet.(Yeah yeah yeah, I know its 3.28 blah blah blah feet per meter but were not putting men on the moon, were just trying to compensate for a freaking crapy torpedo, sorry about that, just know there are rivit counters in the crowd

) All things considered then you should hit between 23 and 35 feet below waterline. You want to hit as close to the keel as possible, but not undershoot completely. Also make sure you get a spread. 20 holes in the same compartment does not make a boat sink any faster. Course i do wonder if you kept the same aiming point yet vary the depth vertically, could you saw him in half?:hmm:
Sorry about the confusion. Oh the horror, there is probably a Blue Whale somewhere in the pacific wanting to know who's got it out for him, and its all my fault