You can buy non-steam versions of SH5 for like $10.
So there is no excuse not to just buy a copy you can easily mod.
For manual TDC, an easy way is the 90 degree fixed-wire attack.
For this attack, range to target does not matter.
You must determine the target speed and course, and then plot an intercept 90 degrees to the target's course.
Set your scope at 000, and turn on the TDC, and enter the target speed in the TDC. Change the Angle on Bow to 90 degrees to either starboard or port. If your uboat is to the right of the target, set it to 90 starboard. If your uboat is to the left of the target, set it to 90 Port. Turn off the TDC.
Now move your scope towards the target. Watch the gyro angle. Keep moving the scope until the gyro angle is 0 (straight up on both dials). This should be 10 degrees or so to the left or right of 0 bearing, depending on your torpedo speed setting.
Now just wait for the target(s) to cross your vertical reticle. As the choice part of the target crosses the reticle, fire your torpedo. You can target multiple rows in a convoy this way. If you do it right they will hit bang - bang - bang - bang from farthest to nearest.
Make sure you open your tube doors prior to firing or you will miss your shot waiting for the doors to open.
Steve