The lead angle was set by the gyroangle, which used the target speed to determine the amount of angle. The faster the target the more the lead. For something like 5-8kts around a 10 degree angle would be good, if your target is traveling 10-12kts something more like 15-20 degrees would be better.
Torpedo speed is tricky. All the torpedo speed upgrades you get from skills and abilities are not included in the TDC calculations by the game, so I strongly suggest not training them, or use a mod that neutralizes them.
This type of attack above is something called the "Dick O'Kane" method, adapted from the US Fleet Boats he captained. The 000 gyroangle just means that the torpedoes are travelling straight ahead and will hit at a 90 degree angle, all preset.
The TDC is powerful enough so that you don't always have to do this kind attack. As long as you've entered in all the information correctly, you can hit from lots of different attack vectors. Just the other night I was on a parallel course with my target, behind him to his right by about 2000m. I put in all the info into the TDC and fired a torpedo.
It raced a head, then curved towards the target's course. Direct hit! I'd aimed at the funnel, and it hit just a bit behind there, but still took out the propellers. Finished him off with the deck gun.
You're correct, this is a pretty good way of attacking a convoy you've gotten in front of. But other angle attacks can be just as effective, if not as easy to setup.
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