4

Best way to find the limit to your tire is to throw some beads. Then you will know. I ran 12 psi on stock tires.

 

I once got hold of a pressure/ loadcapacity list for F load tires from Michelin.
In that for different speeds and for in sand/mud and on track.
I studied this list, and came to conclusion that for mud/sand you can use 50% of the pressure for 20kmu/12.5mh ,wich then is max adviced speed.
For on track about 80% of onroadpressure for 65km/40m/h.
Yes, for lower speed on the road, you can also do with lower pressure.

One wrote already about the 50% of advice pressure, but because of the 12.5m/h it can even be lower.