Wherever the BDC thing (reticule or knob) is, it's only good for one muzzle velocity, one altitude, one set of atmospheric conditions and one bullet. Altitude and atmospheric conditions not being right can easily cause a 5 foot elevation error in shot impact at 1000 yards.
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master