A suggestion

If you are really concerned,

Go to the place where you buy your tires.

Ask one of the guys/gals there to measure your tire pressures with his/her instrument of choice.

Measure the pressures with your gauge.

Make a note of the difference, if any, and use it in the future when you use your own gauge.

The assumption, of course, is that it a reading is good enough for a professional, it should be good engough for you.