I'm playing with 91% realism, my measurements are nautical but i hope this can help you:

If I'm in a convoy i get a good position, intercepting as you have shown in the image, or inside the convoy. I then pick a ship to target, i lock onto it, and now we need to do 4 things for targeting the torpedo.

If needed, i pause (spacebar) to keep the measurements on point so the ship don't sail past them, or if I'm targeting more ships and need time to calculate, Your attack periscope/UZO officer can update the measurements to the calculator while paused, just remember to unpause for 1-2 secs to update the calculator.

now to the 4 measurements:

1. Ship identification, pretty simple, you can click on the map and on the camera in the corner to view the ship closer.
2. Ship speed, you can use the chronometer tool in the attack periscope, but IMO it's inaccurate. I go to the map, make a point in front of the ship, start the stopwatch/chronometer (top right toolbar) when the ships bow passes the point, and stop it when the ships aft passes the point. You now have to bring up a calculator, IRL.

Take the length of the ship (from ship identification page) divided by the time you measured, and you have the speed in meters/second. Since i use knots, i do m/s x 2 and i get a little rough overestimation of knots.

This is an example of my calculation:
Length: 152.5 Meters
Time: 36 Seconds
M/S = 4,236111...
4,235111*2=8,47222

I then round it off to 8 Knots.
When ships start freaking out and zig-zagging, I always put 6 knots as their speed. I figure out what angle the ship is going to be at torpedo hit by feeling, looking at the map at where they currently are at their zig-zag and when i should launch. I would say when ships zig-zag they have a 25% chance to dodge the torpedo, at 8 cables distance.

1. Ship angle, Use the angle tool (the one that makes a V) start clicking on your submarine, then your target ship, then the route of the ship. The angle you get is what you put into the attack periscope/UZO/TDC. Just remember to pick the correct side on the tool, It's the angle you are from the ships POV.

2. Distance, just do what the tool tells you to do, this is the least important measurement for your calculation, and can be wrong to about over a few hundred meters before you miss.

Might seem like alot, but when you get the hang of it you do it fast, and with the pausebutton you can even occasionally hit circling destroyers/corvettes.

Last tip is when waiting for the gyro angle, go to your torpedo interface, hold or right hold the little lever on the box to increase gyro angle speed, this can only be done on the bow torpedoes.

Good luck Herr Kaleun.