If your scope has a total elevation adjustment of 65 MOA (like a Viper PST 6-24X50), it has +32.5 and -32.5 MOA adjustment for the mechanical center. Your 20 MOA rail give you an additional 20 MOA adjustment up, relative to a standard base.
However, unless you have a custom rifle
with a trued action and a very straight barrel, your factory rifle could have up to 20 MOA of error in any direction between the scope base and rifle bore at the muzzle. That error will add or subtract to the 32.5 + 20 MOA that you think you have. You won't know how much up adjustment you really have until you mount a scope and boresight it to the rifle.