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.

"When you disarm the people, you commence to offend them and show that you distrust them either through cowardice or lack of confidence, and both of these opinions generate hatred." Niccolo Machiavelli