Zig-zagging prey

[Kobal2]

As everyone knows, the limiting factor in a patrol is the torpedo load, so I always try to sink ships in one torpedo. In good weather it's quite easy to get a deadly precise solution even at 1000+meters, but in bad weather the range data is sketchy at best, since I can only get quick and unprecise snapshots of the target inbetween waves sloshing over my scope, and the target rolling up and down doesn't help either... I still get a hit most of the time, but on C2s a single hit isn't enough if it's not right in the fuel hold...

And when they you hit them, they definitely know you're here, and start zig-zagging like crazy if you aren't lucky enough to have stopped them.

Now, after a couple of frustrating experiences in which I emptied my stock for nothing, I've got the hang of the timing, I know I should fire when they're roughly 45° towards or away from me and starting to turn back to their original course, so that when the torp actually hits they'll be close to 90° BUT the speed enterred in my solution is no longer valid, since they're turning thus bleeding speed and following a longer route than if they were heading straight.

My question is : what percentage of their original speed should I use ? On a C2 moving a 7,5 knots, I tried 5 knots at 900m, but my torp missed their stern by a hair (I was aiming at the smokestack). What do you guys do ?

(Most of the time I just run ahead 20 km and wait for them, by that time they're back on a straight path, but you can't always do that if they're fast, or if there are destroyers on the way...)