Dividing speed by three is too much, that would mean a ship going 12 knots would only do 4 zigzagging
more like subtract 2 or 3 knots, a ship doing 7 knots will slow down to 4 or 5 usually
I'd also suggest a mag shot set to 1,5 meters below draft and aimed aft, I'd go for the aftmost mast, this means if you're a knot fast in your speed estimation you'll probably still hit. even over a range of 1500 meters or so, beyond that it gets trickier obviously
if you've taking the time to make a new speed estimate with the 3.15 trick, if you aim for center mass and it's turning away from you, don't "lead" the shot too much, if it's turning toward you lead a little more.
or fire 2 eels initially if you want to make sure, although I prefer the one eel one ship tactic myself