I'm pulling a 42' timpte hopper with a 9100 IH daycab empty at around 24,500 depending on fuel.  I usually try for 67pounds on my trailer gauge and 64 or 65 on the truck when I'm all done but in order for these gauges to be accurate it takes a lot of trial and error and a consistent loading surface.  I would not rely on the gauges if I was loading in a soft field.  Also you need to figure it out for your truck.  I have a friend that has a trailer with mini air ride and he will usually read up around 90 psi.