Enter the distance between your scope rings (or the length of a one-piece mount) and how many MOA of extra elevation you need, and this calculates the exact shim thickness to get there. Most shooters use this to compensate for barrel droop on break-barrel spring and gas-piston rifles. For the full picture on droop itself, including hardware options that skip shimming entirely, see our guide to fixing barrel droop.
Measure the center-to-center distance between your two scope rings, or if you're using a one-piece mount, its full length. This is the "Rw" in the formula, real ring spacing, not your scope's tube diameter. A typical two-piece mount setup runs somewhere between 3 and 5 inches depending on the rifle and scope length.
This is the elevation adjustment your scope is missing, not its total range. If your scope maxes out its elevation turret and you're still shooting low, the shortfall is what you need to make up with a shim. Most droop-related shortfalls fall somewhere between 10 and 30 MOA, though severe cases can run higher.
The shim goes under the rear ring or the rear of a one-piece mount, raising the back of the scope slightly to angle the front down toward the bore. A thin strip of brass shim stock, soda can aluminum, or purpose-made shim material all work; what matters is thickness, not material, as long as it's rigid enough not to compress under the ring's clamping pressure.