Take the rings you already have, and put them on the scope loosely. Then set them on the rifle. If the scope isn't hitting the rail, you're good with the rings you already have. If the rings can't mount to the rail because the objective lens is hitting the rail - you'll need higher rings. It's pretty simple.
A 20 MOA base is for long distance shooting. You don't need one.
If the scope fits on with the rings you already have....you're all set.