Your first step is to chronograph your rifle/ammo combo then input that reading into your gun profile (MV) along with the other input variables. Be sure that your inputs are accurate ex. SH, BC, ZR... If your ballistic Firing solution isn't matching up with your actual drop (what your physically seeing downrange) then you want to conduct a calibration on the variable with the greater uncertainty and through the supersonic realm of flight that variable in muzzle velocity. Under GUN, scroll down to Cal MV and hit center button. On the MV Cal screen you will see a range in the top right corner, that is the recommended range to conduct a MV true. Simply set up a target within that range +/- 50 meters/yards and record the actual drop needed to hit that target. Be sure that the calibration target range is confirmed with an LRF and take caution on measuring the drop, any error will lead to an improper calibration. Once drop is set simply hit the Cal and then hit back button to accept mv Cal. The new MV will be used.