1. Spinning Balls Diameter
2. Ring Internal Diameter (I.D.)
3. Ring Gauge
4. Internal Diameter Minus Ball Diameter.

For an I.D. of 30mm (a) / ball dia of 8mm (b) / gauge of 4mm (c). number 4 will be :-
a-((b-c)/2) .... or .... 30-((8-4)/2) = 28.
Some people do prefer the I.D. to be a 2mm tighter than the erect diameter of the head, to stop excessive movement of the ring.