i had the exact same problem a few weeks ago & have had it in the past..
as said, find the middle setting of the turrets, then try & find your zero, when you find it & you either notice that you cant get on target or that its on target with only a few clicks left, then go down the route of using shims, but personally i would use thin card..
for example, if your shots are hitting 3" high from where your cross hairs sit, you would need to put a shim on the front mount where the scope sits.. ..
let us know how you get on..