Ehr, not sure how you're targeting, but if you're firing on 0 bearing using the scope set to a predetermined angle, with your course at a right angle to target course, you should leed by 13 (12.8) degrees instead of 10... at least if my trigonometry is correct, which it may not be.
14 degrees for middle speed, 18.4 for low speed. Of course, that all goes out the window if you upgrade your torpedoman (boosts torp speed).