Still relies on the eye and judgement. You may do it well because you are practised. I can’t imagine an amateur getting it that accurate.Michael.N. wrote: ↑Sun Feb 18, 2018 12:08 amYou can use a 6 inch metal ruler to measure action. I actually prefer them without the 0.5 mm increments. I just 'split' the mm by eye. If you go up to the 14th fret, down to the 10th and back up to the 12 th you should be able to read 0.2 mm accuracy if not slightly better. More than good enough.
There is a simple way involving taking a high resolution photo from a camera or camera phone at right angles to the neck and between the plane of the strings and the fretboard. If you know the scale of the guitar then you already know the fret to fret spacing between the 11th and 12th frets or even the 11th and 13th frets. Blow the photo up on a PC or iPad and use a ruler to measure distances and you can calculate the action pretty easily just using measured ratios and a known fret to fret distance.
I know this technique presumes accurately installed frets, but I am hopeful a luthier would do this pretty accurately - certainly more accurate than the error estimating action height by eye.