i run my 36's at 12psi in the summer on the pavement.

as for the rating on the sidewall, that is for a full size truck that runs that size tire stock. think unimog.
we're talking a jeep that is less than half that weight.

also, remember that when you are zipping down the highway, your pressure will increase with speed. if you have too much air in there combined with highway speeds, your tire will crown. eventually, you'll lose tread depth in the center.

you can do the chalk trick by rubbing chalk across the width of the tire and drive in a straight line at different speeds to see how the pattern wore, then adjust as needed.