That is then perfectly true in USA that calibration of the gauge is not required in USA - but as per the AWB and the REG,s it does need calibration.
If you had a letter the gauge is required to be calibrated only once or every 500 years then you use that data - but in the absence of that you need to use the fall back of the CAsA AWB or higher.
I have the same rig and the gauge was required to be calibrated due the 35 psi difference in spark compared to a new plug.
* none of that Champion Spark Plug Tester (125 psi) stuff is relevant to this tester - it is a different test to confirm spark plug serviceability.
* topics like this being other reasons I closed my doors - I agree with you it is BS but by the REGS and associated publications - the gauge needs to be calibrated.