The G3X does do the LOP %, so that is based on fuel flow, and I would guess 8.5:1
For the purposes of knowing roughly your %HP near enough is good enough.
When ROP, it works on Mass air flow, basically MAP and RPM are it.
With any engine monitor, the simple test is go up and fly full rich, at a steady MAP and RPM, then lean all the way to LOP and to ICO. The old JPI's and even the G1000 (I think) would still show the %power of say 75% all the way through to ICO, so long as the MAP and RPM were constant.
Ohh and if you are worried, just run the mixture back up, dont touch anything else. It will relight again.