If you'll look thru your scope at a distant object, and then turn your power ring up, you'll notice that the relationship of crosshairs and the size of the object changes. This is true on nearly all scopes. This is important if you're using the top of the bottom post to aim with because at the lowest power setting, you may be aiming several feet higher than at the high setting. This is how most of the BDC, Boone-and-Crockett, etc type scopes work. You have to shoot to learn at which power the yardage marks correspond to how your rifle shoots. Otherwise, each scope would be caliber/load specific. On my Rem 700 in .280, I have a Nikon Monarch with BDC. It happens to be dead on with the loads I shoot when turned up to 9 power. When I center the 400 yard circle on the 13" plate, it has never missed the plate. Might not always hit dead-center, but it hit the plate. I have two Burris scopes with the Ballistic Plex reticle, which is the same way. They work perfectly at 250 yards, though I have not tried them at 400 yet.