(I posted this here awhile back. Seems my exact targeting/intercept method has evolved a bit, but the general ideas are the same.)
"It's not hard, just takes practice.
FWIW, here's what I do.
1. Determine a rough course. Draw rough course with ruler out to way past where I think my intercept point will be. Start heading towards rough intercept point.
2. Determine speed - Mark an x and start the stopwatch. At 3 minutes 15 seconds mark another x. Measure the distance using the compass tool. This distance*10 = speed in knots. (e.g if the distance is .8 km then the speed is 8 knots, 1.2 km then the speed is 12 knots, etc.)
3. Optional 3rd mark at 6:30 to better nail down the speed.
4. Now that I have 3+ marks, I'll draw a "tighter" target course with the ruler, layed out at a distance to correspond with the targets speed. (e.g. If he is running 8 knots I'll mark out a 24 km course. That way I will know that in one hour he will be at the 8 km mark, in two hours he will be at the 16 km mark, 3 hours at the 24 km mark, and so on.)
5. Plot an intercept course and speed (Dantenoc's tutorial on this in the community manual is excellent) to put me 600 meters off the target's track at a 90 degree angle.
6. While I'm running to the intercept point, I'll enter the target's speed, the pistol type, the depth of the torpedo and the speed of the torpedo into the tdc. Bearing will be 0 as I am setting up for a 90 degree shot. I'll also put in a rough angle on the bow (usually 90 degrees port or starboard).
7. As the ship nears, I use the protractor to firm up my angle on the bow, (first point at the target, apex along his track, last point on my sub), and check his speed).
8. As he closes to 0 degrees, open the torpedo tube doors and pop up the periscope. When the point I want to hit crosses 0 in the periscope, Los!
9. Kaboom!
(Thanks to all who took the time to document all this, create the video tutorials, and share it all with everyone. In a good couple hours of playing with it all I got it nailed down pretty well.)
Now I'm experimenting with a 10-15 degree offset to make the torpedoes hit even closer to 90 degrees, thanks to some tutorials in the SHIV forum.
Y'all rock.
"