Another option is to lap the bases. Sounds like the rear one needs to be lower. Depending on how much height difference you need I would remove the bases and wrap the action with 220 or finer grit wet or dry sand paper and stroke the base over the mounting area lightly. Look for full contact and remount to check alignment frequently. This could reduce the need for shims or possibly make them unnecessary. Depending on the type of mount you could also remove material from the top of the base but harder for this not to show although cold blueing will make it not so apparent.
There is a formula for calculating how much the mounting is off but I would have to look it up, it is for iron sights but may adapt to scopes. Or you could shim till the scope is centered and use that as the measurement. But if it takes more than some tape or a single shim I would turn it over to a good gunsmith.
You need to be careful with the lapping. Even one thickness of sandpaper increases the radius of curvature of the action. If you fully lap to that radius, when you remove the paper, the base now has a larger radius of curvature than the action. The base will make contact with the action only on a line between the screw holes. Then you will need to epoxy bed the base to the receiver.
The formula for scope shimming is the same as for sights, but you use the distance between the rings instead of the distance between the front and rear sight. As mathman mentioned, similar triangle is the way to think about it.
