I think you need to practice with existing numbers (1, 2, 3 circles you're talking about, right?), they seem to be quite sufficient and I hit nearly every target; it almost gets boring how ACCURATE it is. I use fast torpedo settings, the number 3 corresponds to something like 1800 meters/yards, that is pretty far distance already, and I'm hitting targets no problem, unless it's a destroyer doing sharp turns at 35 knots flank speed...
I hope I understood you question right, sorry if not.
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