You should be able to calculate the right tire pressure by cross referencing the chassis weight specs with the tire specs.
I run 85 in the front and 90 in the rear. The front of my 2007 F53 is spec'd to handle 7,500 lbs out of the 22,000 GVWR.
I have Michelin XRV tires. The XRV tire chart pegs 85psi at 3950 per tire, so I'm actually over by 400 lbs in the front (200 per tire). (3950 x 2 = 7900. 7900 - 7500 = 400)
90 psi in the front would put me over by ~700 lbs and for me that's overinflated.
90 psi in the back (given dualies) gives me 15,060 lbs per the Michelin chart. The chassis is set up to put 14,500 of the 22,000 in the rear.
So in the rear I'm over by 560 total or 560 / 4 = 140 lbs per tire.
So I could weigh it all the time and putter around with the pressure to get it just right, but I assume that the weight isn't even and so I leave them over a little bit and don't worry about it. I would rather be a little over than under.