1. The ISO standard for calibration of reflected meter has a manufacturer chosen Constant -- essentially this is a VARIABLE value, selected by the meter manufacturer.

2. The ISO standard for calibration of incident meter has a manufacturer chosen Constant -- essentially this is a VARIABLE value, selected by the meter manufacturer.


...so the fact that meter #1 does not match meter #2 is inherent to the ISO standards for the two meters!

3. Although 18% grey tone is 'middle of the tonal range', one can determine that really one needs to meter 12-13% grey card, for #1 to truly match #2 in reading.