Each model scope will be different because lengh plays a role with a tapered mount. Mount taper angle also changes things. Either the scope mfg. or the mount/ring mfg. should be able to tell you which will be the best fit and what mount taper the scope was designed for. 20 MOA is usually enough, but some scopes are designed for 45 MOA. Using the later may be required if you want to use all of the available elevation travel.