I was reading through the manual (infact i had posted a link to those sections of it onto the board just before) to confirm this when i realised that you will still get the shot you want with this method. I'll post a couple of annotated screenshots as I test it.
UPDATE Here is a snippit of the relevent part of it:
810. ANALYSIS OF TORPEDO FIRING:
(a) Straight Fire
Torpedo firing in which small gyro angles (less than 30 degrees) are used is considered to be "Straight Fire". The curves plotted on plates XVII and XVIII for 46 and 29 knot torpedoes were developed by plotting the deflection angle against the torpedo track angle for different target speeds. It should be noted that in all cases the gyro angle was zero.
(b) The slope of these curves at any point is the instantaneous rate of change of deflection angle with torpedo track angle. The optimum torpedo track angle for any given target speed is the torpedo track angle for which the rate of change of deflection angle is the least. This is indicated on the curves by the shaded areas.
8-8