When the gauge is all heated up and the reading is low, try pushing the cap to the side and letting it vent.
See if the needle moves.
Supposedly, a temp change of 30-35* can change the reading 1psi if it’s not vented.
I can see the under hood temps changing over 100* in the area where gauge is located.
Thanks I’ll give that a try when I get this thing running better. At least for now I know that an initial rating is probably good enough. But at some point, it would be nice to be able to see true pressure all the time.