What Afkirby said is accurate for many scopes from the mfr, they set the elevation at center with the expectation that you will re-zero once you get it mounted. Think of it this way, if the mfr spec's a scope with 30mil total elevation that means 30 mils from the bottom to the top; however, if you mount with 0 cant you will most likely only get 15 mil of adjustment from zero, put the scope on a 20moa rail or mount and now you have to re-zero to adjust for the almost 6 mils of cant, once you do that you now add the 6 to the 15 and should have about 21 mils of adjustment. This is based on perfect world but in the real world your actual adjustment may fluctuate a bit, but as long as your close to what the math says you're good. I had a situation last year where I had a 20 moa rail but was only getting about 14 mils of adjustment when I mounted my AMG on it, I thought maybe I had a bum scope at first until I realized that my 20 moa rail was actually a 0 moa rail (that's what I get for buying a cheap rail), once I put a Nightforce 20moa rail on the action my AMG suddenly had 20 mil of elevation.