Truing
Adjusting the inputs to a ballistic calculator until predicted impact matches observed impact at distance. Calibrates the model to the rifle.
Even with perfect velocity, BC, and atmospherics, a ballistic prediction can be off by a tenth of a mil or two at 1,000 yards. Truing fixes that by tweaking the velocity (or BC) input until predictions match actuals across the distance range.
Modern ballistic apps (Hornady 4DOF, Strelok, Applied Ballistics) have built-in truing workflows: input your dial-and-observe data from 600, 800, 1,000 yards, and the app solves for the velocity that best fits.