I think you must take AOB into consideration. If targets speed and distance from you is constant in your query, all you got to do is choose some AOB (likely 90) and bearing with that AOB, and then check with TDC. Set torpedo speed to slow, check on what bearing you get 000 gyroangle, change torp. speed to medium, repeat and then change speed to fast and check again. Write everything down, you can get all needed information in couple of minutes, using imaginary target.
This is the simplest solution, or I misunderstood your request totally