On the typical american scope (almost all are second focal plane reticals) you actually change the power setting of the scope. I have a Leupold VXIII4.5-14 with the B&C retical on my 300wsm, it calibrates pretty much bang on at 14X, with my 165 tsx handloads. I tried it on my 308 and it is dead on at 12X, I made a little white dot at that location with a white-out pen.
My VXIII came with 2 settings marked on the magnification ring and in the instructions it gives you some guidelines as to which calibers use each setting, but this needs to be confirmed at the rifle range. If you are shooting 500 yards with the 500 yard hashmark, and the bullet hits low you decrease the power setting on the scope, if it hits high you increase the magnification.