Not all decks are uniform like a Velocity deck. You may measure from a consistent position to check and adjust for level as the Velocity deck is a fabricated deck with a flat top.
As I have dealt with many different types of decks, I do the actual blade height.
Part of my garage floor is flat and level and I do adjustments on that area. Not everyone has a flat and level area to make these adjustments.
On every deck that I setup, I adjust so the forward blade tip is 1/8" to 3/16" lower than the rear tip and sometimes more as the operators manual may dictate.
This way the advancing blades start the cut of the grass from about the 9 to 11 o'clock position, following thru and discharging properly out the R/H discharge chute.
Scag and many other ZTRs hang the deck from the machine frame. Others use rear and sometimes front guide wheels that roll on the lawn surface and the guide wheel adjustment is the critical adjustment.