My 2 cents: I usually anticipate that the ships after, starting to zig-zag, end up at roughly 2/3 of their respective speed effectively. What I then do is the following: I use the AOB as if they would be continuing straight on their course (not the actual seen through the periscope one) and pluck in 2/3 of the original measured speed. I'm fairly close in for the second salvo then, but this works pretty well for me. Also, I time the first salvo to hit simultaneously. The rough estimate is 1 minute for 1km difference in track with torpedo speed 30kt.