Just keep in mind that the attack disc itself could not compute this, it only ever showed relative angular relationships. The reverse side however, which is not historical to the Germans, is a logarithmic slide rule that allows you to compute trigonometry problems such as this. It should be said though that computing lead angle for a perpendicular attack using that slide rule is somewhat laborious, because it does not include a tangent scale. Sub Buddy app, however, does (highly recommended).
Something like this is very easy to calculate with a calculator. For a perpendicular attack, lead angle = arctan ( target speed / torpedo speed).
So on a calculator take target speed divided by torpedo speed, and then press “tan-1”.
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