Parallax correction?
 

Under what circumstances must the parallax correction be added to the lead angle to calculate the shot angle and when must it be subtracted?
Can someone please explain this to me?

Eh?
 

I don't think people have understood your question. In SH3 the TDC calculates the torpedo gyro angle including the parallax error caused by the distance from the periscope to the torpedo tube and the distance the torpedo requires to turn onto the setting of the gyro angle. This is why you need to enter the range if the gyro angle is not zero.


Calculating "Aim off" is something you would do without a TDC.


If you need to understand it there's lots of complicated sums here:


http://www.tvre.org/en/gyro-angled-torpedoes