First off...don't get all hung up on scope height. Assuming a 300 yard zero. If I am off by 1/2 inch...say 2.0 instead of 2.5 inche scope height.....a HUGE error, my impact only changes 1.2 inches at 1000 yards.... 1/10 moa!!
So don't sweat the small stuff.
Now, that being said..I STILL like to know.
So....1/2 diameter of action. That is easy......Take remington 700's and 40XB's...1.35 inch. Divide by 2 = .68. Just measure your action or look it up and divide by two.
1/2 scope TUBE diameter say a 30mm tube... 1.18 devided by 2 = .59 inches.
Measure the distance from the bottom of the scope base (where the base contacts the top of the action) to the bottom of the scope ring where the bottom of the scope rests in the ring..... lets say for this case is 1 inch.
So.... .68 + .59 + 1.0 = 2.27 inches.
If you have a 20 , 30, or whatever MOA base, and you are truly, truly ANAL......measure BOTH the front and back ring/base height and divide by two and use that number. But really.......come on!!