Nothing to do with speed, it's all about firing position. The torpedo speed would affect when you fire, but not the ideal firing position. Looking at don1reed's chart, as an example you first sight a ship 6000 meters away, heading southwest. Range is 6000, angle on the bow 30. You look at the 30 in the left column, trace across to the column under 6000, and the distance to the track is 3000 meters. You want to be within 1000 meters, but if you turn directly toward the target and try to close directly you'll end up at a bad angle. You could also set up what the aviators call a pursuit curve, running fast and leading the target, but then you still have no way of knowing if you're leading too much or not enough, or running too fast or too slow. So you turn your sub northwest or southeast (depending on which side you're on) to get at right angles to the target's track, then close on the track. When the range closes to 4000 meters, you again check the angle on the bow. If it's increased to 50, you're falling behind and need to speed up, if it's still at 30 you're 2000 meters from the track with 1000 meters to go, if ithe angle on the bow is down to 20 you're 1368 meters from the track and should slow down.
The beauty of that chart is it considers only angles and distances, speed of the target is irrelevant since the angle to distance formula works at any speed.
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