Search the subsim downloads for Angriffscheibe Handbuch. It should come up with a community created PDF in RAR archive format. It explains how you can do various calculations with it. (can also be found in the bottom link in my signature)
Just be aware, across-targettrack/perpendicular or 0 gyro angle is not a requirement to get torpedoes on target. Just that is makes the best odds of succes. And are in no way actually part of the lead angle calculation. Only target speed, torpedo speed and AOB through the sine rule : opposite side/sine (angle)=constant
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