Measure string action (the height of the string above the fret) at any point by placing the gauge behind the string. The string height markings are at increments of .25mm (1/4mm). When the bottom of a mark aligns with the bottom of the string, that measurement is the string height at that point.
The markings themselves are .1mm thick, so referring to the top of a mark instead of the bottom adds .1mm to the measurement. In this way the progressive string height scale measures in .1mm increments.
String action specs
The suggested measurements listed here refer to the string height at the 1st fret and also at a higher fret. Measured for the outermost bass and treble strings.
Lay a straightedge across the frets and measure the clearance at the 8th. (Or
put a capo at the 1st fret and press a string down at the highest fret —
the string becomes the straightedge.)