PSI, tire ratio (e.g. 60, 65, 70, 75, 80 series) and type of tire (e.g. A/T, M/T, side wall ply's, etc.), vehicle weight and terrain all impact how low you can air down.
There is not any one optimal pressure setting that can be used on all tires.
Generally a shorter side wall (40, 50, etc. series) tire, you won't be able air down very much.
The higher/taller side walls, and depending upon how many ply's and type of material the ply's are made of,
and if your tire is a "light truck" rating, you should be able to air down to a much lesser pressure.
Rock crawlers can usually air down to sub 10 psi without bead locks. With bead locks, as low as 5 psi.
To recap about airing down.
Have as small diameter rim, for our GX's that'll be 17", AND as narrow as possible rim, like 8" - 8.5". ALL off roaders (who know anything about off roading) do this. A narrower rim will keep your tire from blowing off the rim lip when you air down.
A smaller rim, as mentioned above will allow a much larger side wall. A larger side wall is like having an extra long travel shock. It gives more cushion travel, hence a smother ride.
A "small truck" tire will have a much stiffer side wall and be able to air down to at least 12 psi. (Ask how I know...)
Depending upon how many ply's and what material (Kevlar, polyester, etc.) there are in your side walls, that too will impact how little pressure you can run.
So with all due respect to all the people posting what PSI to run, please consider what I've posted.
Here's a reference:
The Best Tire Air Pressure for Off-Roading | Inside Line - https://www.drivingline.com/articles/the-best-tire-air-pressure-for-off-roading-inside-line/