400 to 450 is what I've found so far too. The most common number is 450. I think LukeFF had the same results in his research. Even with this setting, a couple of boats had the safety fail and the torpedo detonated a couple hundred yards from the boat. Then there are the boats where a fish stuck in the tube.
What tater talked about is called "countermining" and the delay isn't just for premature detonations. If the spread is small and the firing interval too short, here's what could happen in real life. The explosion from a hit by the first fish of a 3 torpedo spread can cause the second torpedo to detonate away from the target, which may or may not be close enough to do any damage, and the 2nd fish exploding which will in turn cause the 3rd torpedo to detonate- which will be too far away to do any damage. (Never checked to see if countermining happens in SH4, hmmm.....:hmm
. At any rate, same thing can happen with a spread of any number of fish.
Probably one of the reasons you don't see a salvo button for fleet boats. If you can't use the keyboard to fire, move to the next tube, and then use the mouse to change the spread angle in less than 8-10 seconds, you need some practice.
With all mouse movements you should still be able to do it in less than 12.