This is an excellent resource to calculate ring height - http://www.mil-rad.com/scope_ring_calculator
FWIW, don't try and get a mm of clearance as the inherent flex in the optics/mount/firearm can cause the optics to touch on firing.
As a point of reference, I used the above to calculate that I needed the Seekins High 34mm rings for my RPR 6.5. Clearance was perfect.