A 12 inches or 18 inches stainless steel rule accuracy depends only on the ability of the individual to read it.
I know many people that cannot read a regular ruler to 1 /16 of an inch.

We can calibrate measuring instruments to a satisfactory accuracy against a known referecnce.
So we can check the calibrationn of an incidence meter against a physical set up done by mathematics.

I certainly realize that many hobbyists just hate mathematics and evendislikepeople using them thus some comments in the last few postings.

My feelings are that calibration needs to be accurate only to a practical accuracy for the task being done.

Zor