I may have missed your point!
That scope has about 56 MOA of vertical adjustment.
Where is it set now?
IMO, for maximum long distance shooting it should be near the bottom DOWN adjustment.
Personally I would stay at least a few clicks off of bottom to be kind to the mechanism.
In a perfect world that should mean that you need about 25 MOA+- of cant in the scope.
If I guess that your base screws are 4 inches apart you would have 3600 / 4 = 900
Round it off to 25 inches + about 2 inches of drop = 27 inches
27 / 900 = 0.030 shim
The world is rarely perfect, but you have a shim in place that "should" have gotten you close. It seems strange that it should be off by 20 inches....unless your scope is not adjusted fully or the distance between base screws is off wildly ( which I only guessed at