Since there is disagreement about Zero Gyro Angle shots I thought i would provide some background.
The Submarine Fire Control Manual has a hint to the math involved
BLANK 840. DEFLECTION ANGLE FOR STRAIGHT FIRE:
The deflection angle for a straight shot of any torpedo run for target speeds less than one-half the torpedo speed, may be approximated as follows:
Torpedo
Track Angle 46 Knot Torpedo 29 Knot Torpedo
1. 90 degrees 1 1/4 x Target Speed 2 x Target Speed
2. 60 degrees 75% of (1) 75% of (1)
3. 40 degrees 50% or (1) 50% of (1)
4. 18 degrees 25% of (1) 25% of (1)
5. Optimum 1-1/3 x Target Speed
In my example I use a 10 knot target and 46 knot torpedo. The approximation suggested in the STFCM is 1.25 times the target speed for a 90 degree shot or a 12.25 degree deflection angle.
The Arctangent of 10/46 is 12.26477 degrees.
All of this is based on well known trigonometric functions.
A torpedo shot is a simple time and distance problem.
You choose an impact point on the target course line. You position the boat at a firing position within range of the torpedo. You must fire the torpedo so that it arrives at the impact point when the target is occupying the impact point.
That time is determined by the ratio of the torpedo speed to the target speed.
A 46 knot torpedo travels 4.6 times as far as a 10 knot target for ANY given distance.
For any time (T) the sides for this triangle are 10(T) and 46 (T). Thus we can ignore the T.
So far ANY target range the ratio of target speed to torpedo speed is constant, thus the firing angle is constant for any range.
From the STFCM you can see the rule of thumb applies to any shot within 30 degrees of "Normal" (The course 90 degrees off the target course)
So any time you can get into a firing position that allow a nearly 90 degree torpedo impact to target course the trig table I posted is extremely accurate ASSUMING you develop an accurate target speed and use the correct torpedo speed.
No messing with the TDC is required for Bow tubes in SH4.