To determine the ring height which will miniize the barrel to bell separation you also need to know the diameter of the barrel at the point where the scope bell passes over it, the diameter of the receiver, the height of the base, and the minimum acceptable separation. But the advantages of having the the scope centered on the eye when the rifle is properly held far outweigh any advantage of having the scope closer (or farther) from the barrel. Of course the scope should not touch the barrel including during recoil and should not interfere with feeding and ejection. Raising the scope and adding a cheekpiece is sometimes a desirable solution.