Just so we're using equivalent units of measurement, the thickness of a standard credit card is 0.76mm. Measuring relief between the 1st and 24th fret will give a higher value than from the nut to the 12th fret. It usually works out that relief of 0.13mm - 0.20mm measured between the nut and 12th fret will give relief of approximately 0.60mm - 0.76mm when measured mid point between the 1st and 24th frets. In other words, unless you are stupidly fussy, the credit card thing works pretty well.

What he said!
