You really need to maintain ring alignment.Just raising up a ring or base with shim stock will mis align the rings.
You pay a lot of money for precision tolerances inside the scope. You might even have some super glue assemblies.
Tighten up mis aligned rings and you will distort your scope tube.All that precision is gone.IMO,that is root cause of a lot of scope trouble.
IF you have a one piece ,bridge or rail type base,you might make do with a shim under one end. I'd supplement wit some form of bedding compound.
Gently snug the screws,let the bedding set,then finish tightening the screws.
emcon is right,the tapered bases are best.
I have never used them,but I think Burris offers some kind of eccentric sleeved rings.Those might get you through your match.
The tan of 1 deg is approx. .01746. One deg is 60 MOA
So,if you want 20 MOA? Divide by 3. About .0058 . Now,how far apart are your rings? Maybe 4 in on a .22? So we multiply 4 times .0058 would give you
a .023 shim,approx.
That .01746 number is key.For every inch of ring spacing,you need .01746 of shim to get 60 MOA on target. So,(ring spacing,in) times .01746 = shim for 60 MOA (1 deg). If you want 15 MOA,(1/4 deg) divide that number by 4.
You will likely be looking at a 1/64 in or a .016 or .018 or .020 shim.