I play the game 100% realistic and only try the 90 degree position, use the stadimeter to calculate the distance and the clock to calculate the speed, then move the Periscope until the Gyroangle display with both arrows is pointing upwards and you know when to fire the torpedos, the rest is guts feeling. But by now I get the targets by 90 percent.
Save your torpedos normally 1 is sufficient (destroyers smaller ships and tankers) sometime two. If there is no destroyer aroind you can surface and sink it with you deck gun. Take care to keep distance as they might be able to fire back...