Take the diameter of your scope tube + diameter of bolt ( on an open top type action) or action on a closed top type and divide that in half. Add in the distance between top of bolt( action) and bottom if scope tube. That is the #. This data does matter as distance increases.
As for ROT you can do it the old fashioned way using witness marks and a tight patch on a rod. Mark the rod at your "start" and your "finish" of one full rotation. Measure between marks and that's your ROT.
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