errr... not sure about that video. One knot is 0.52m/s. Am I losing the plot?
What I do is:
Time how long it takes the target to traverse the reticule in the periscope.
Take the target length in metres and double it.
Divide that number by the time in seconds and you have knots (near enough).
Robert is your mother's brother. Even I can do that in my head to the nearest knot. And it's historically accurate!
Or you can use the more realistic U-Jagd stopwatch if you have that mod.
If your own boat is moving it will screw up the target speed calculation. Assuming you are travelling towards the target it will actually be going faster than calculated. Some folks have tables to correct the value, but if I'm going at around 2 knots I just add a knot.
If you fire at around zero gyro angle the range does not matter except in calculating time to impact. The TDC will not automatically update the range, so enter what you plan the range to be when you fire.
The difficult bit is the angle on bow... I'm usually perpendicular to the target course, so I know how many degrees I am from having the target straight ahead at 90 degrees AOB. i therefore enter 90 degrees less the number of degrees to go to straight ahead. There are various in game calculator disks to do the job, or you can measure from a plot on the map.
After entering the values, and with the TDC again locked to the periscope, the TDC will keep automatically updating the target relative bearing and AOB, and hence gyro angle, based on the periscope angle. in theory you can fire at any time, and can even "point and shoot" at other ships at the same speed and heading. Unlike the real TDC, the game TDC will not compensate for a change in uboat heading; so do not turn the boat after locking the TDC.
SH3 merchants have captains with cat-like reflexes and can accelerate and brake like F1 cars. So avoid using steam torpedoes in daylight if possible and if attacking several ships try to get all your eels to arrive at the same time.
Most importantly the trick is to get within 1km, but over 300m. That can be a bit suicidal with late war convoys.
Info about the real TDC here:
http://tvre.org/en/handbook-of-the-u-boat-commander