Aw gee, don't make it complicated.
If there's not a lot of shimming to do, and it's not a loudenboomer, a pop can and scissors are your friend.
1. To get an idea of HOW MUCH shim, measure the distance between rings, get out your calculator, divide 100 yards (3600 inches) by your rings. You want seven inches (thats 14 inches at 100) PLUS whatever it takes to get centered, so you should spin the turret to see what your total adjustment is. Half of THAT gets you what you need to hit optical center, which is what you want.
2. You need to decide whether to shim INSIDE the ring, or UNDER the front base. It might be best to get a couple of slightly-longer base screws and shim underneath, because that doesn't mess with the inside diameter. If your total rise is more than one turn of the screw and you have less than three turns, you definitely need to get some longer screws to use all the thread you can.
3. In all cases, it's good to at least CHECK ring alignment and whether or not lapping is needed. If the alignment is bad, I'd shim the bases with longer screws, then lap it good. If alignment is pretty good and you don't need a mess of shims, just plopping a shim about 1/2 wide on the centerline should work okay just as long as you don't crank the ring screws too hard.
I have done:
1. Shim rings, with a skin-coat of JB Weld as an adhesive, then lapped that. You'll need some kind of release agent on either the scope, lapping bar, or whatever straight round stock you use. I also ruff up the inside of the lower ring to give some "grab,"
2. Shim base, again using JB as a gap filler. Lap rings after. If you put JB or some other good epoxy on both rings, you might not need to lap at all. There's also some Loctite green stuff for bedding, Mic McPherson wrote about that. Works for bases, I don't think he's used it on rings, and by the way, he's a huge fan of the Burris setup.
