My CycleMate CM2000 has 8" wheels and the manufacturer says to use 20-25 psi in the tires. The sidewall rating is the maximum pressure for the maximum load of the tire... if you use that for a 300 pound trailer, you'll wear out your tires in a hurry.
What I do is with the trailer loaded put water in front of tires and pull trailer through it play with presure till a contact patch of the full width of tire shows that is the presure you need.:thumbup:
I have a Mini-Mate camper and ran 30-32 psi on my 6000 mile trip last september. I have determined that I need to lower the pressure, to about 25-28 PSI for the next trip. The tires are still in good shape but I can see that the center wore more than the rest of the tire.