Yeah, shim the FRONT base.
I've had to do several. I get KS brass shim stock, but a pop can will work, too. Easy to cut is good.
Then center the scope, right? Fire for error (if it is bad, you might need to be at 25 yards and calculate for 100). Run the calculator as Ron suggests, do everything in inches.
Once you have a basic shim thickness, you can cut a shim for under the base. I usually epoxy the shim to the base with release agent on the receiver.
Next: If you have a one-piece base, it's probably a smart idea to bed front and rear with epoxy and mold release.
If front and rear bases, you should lap the rings for alignment. Or you can cheat with one layer of blue tape in the lower half of the rings. That will get you shooting, but doing it "right" for the long term, you should lap.
Finally, the quickest and dirtiest way is to put the shim stock in the front ring, just about 45 degrees worth on the very bottom. Works great when you DROP YOUR RIFLE ON THE GROUND AND BEND THE SCOPE 300 MILES FROM HOME. Don't ask me how I know this, but it's amazing what you can do with a pop can.
