what is considered a high action measured from the top of the 12th fret to the bottom of the string?
Mine is about 3mm. If it is too high you can just tell, not sure what the magic number is.
I keep mine as low as I can without it fretting out. On my old crappy starter, it was pretty high, but on my RGT I keep it low as hell so I can play more efficiently.
its considered too high when its uncomortable to play
2,1 mm for the lower "E" string...
timeo abstemios et dona ferentes

- lovermate: fender american stratocaster sunburnst
- powermate: marshall mg 30 dfx

- seiko tuner (sooo useful)
- korg metronome (keep fingers bleeding...)
It depends on the type guitar style. What would be extremely high for metal could still be quite low for say slide guitar. It is a matter of what is comfortable.
2mm, which, according to my elitist dad is ridiculously high for an LP. Git. I prefer it that height tbh as it allows bends without hitting other strings off the frets but isn't stupidly high so that its unplayable