I played with manual targeting with map contacts on for years. I think this is a fine way to learn to operate the TDC while having the benefit of a "god's eye view" of the tactical situation on the nav and attack maps.
With map contacts on, it should be relatively trivial to set up a 90 degree attack approach and also exactly determine target speed and heading.
Raise your scope, lock the enemy ship, and it will show up on your maps. You can even lock to them in poor visibility when you cannot actually see them!
Then you just zoom in on them and draw a line through them and you get a nice target course track to intercept. Draw a mark on them and another one on them 3 minutes and 15 seconds later and you have their exact speed.
At this point, you just drive 90 degrees to intercept and set your TDC Angle of Attack to 90 degrees starboard or port (depending on whether your sub is on the starboard or port side of the target) and then move the scope left or right until the gyroangle reads 0 degrees. Then fire when the target crosses your vertical crosshair.
Things don't get really fuzzy until you turn off map contacts.
Steve