Something else to note that's bitten me in the past, when truing (or getting any ballistic solution for that matter) you have to input the line of sight range and shot angle into the calculator so it solves for time of flight, angle modified range is not accurate and the error gets amplified the farther and steeper the shot angle. Not unrealistic to have to adjust velocity a little bit but typically when all my inputs are spot on and I use the box BC and chrono velocity everything lines up damn close.Thanks, I had adjusted my velocity in my calculator when my shots weren't lining up at 800 yards but then had someone suggest changing my bullet bc instead. Was not sure if it mattered or which was correct. I had originally dialed to 6.3 mils at 800 and had to back down to 5.7 mils. Which was about 120 fps velocity change in the calculator.
Also, known repeatable scope? Might be worth doing a tracking or tall target test to be sure there's no error there as well.