Azazle,
This all just for fun, but I'll take a crack at explaining it. You already know that halving the magnification doubles the spaceing between the hash marks, right? So here's the thing, I want a mildot spacing to grow into a MOA spaceing. One mil is 3.6 inches at 100 yards, but I want it to measure 1.047" X 4 = 4.18" (4 MOA) The ratio between what I've got and what I want is 3.6/4.18 = .859 Since the relationship between the magnification and subtension is inverse; 25 X .859 = 21.49 Set the power ring of a 25 X Mark 4 to 21.49 power and the spaceing is now 4 MOA. 'Course, for my uses a 4 MOA spaceing isn't widely useful, but a 1/2 Mil spaceing like a TMR gives a 2 MOA spacing that is some good to me. Dial elevation and hold wind in a pinch.