There are very accurate gauges which can be calibrated. They're usually only used by race teams and engineers due to the price. Sometimes you are lucky and get a cheap digital unit that is dead on.

I have some of those cheap gauges. I compared them to a gauge owned by a tire expert/engineer. One was very accurate so it stays in my tool box as a reference gauge to everything else I use. I even have some stick gauges that compare surprisingly well and are more accurate than some of the cheap digital units.

In truth, for our use, 1 or 2 psi isn't going to be a major problem. Using proper inflation procedure and even pressures puts you ahead of probably more than 99% of the motoring morons...I mean public.