We run about 20-22 psi for general riding, and about 24-25 when on a trip, leaded with the Bunkhouse in tow. It is surprising how much difference a few pounds of air pressure can make.
Years ago I ws told that the way to fine-tune the air in the bike tires is to fill them to spec, then go on a good long ride. Measure the psi, and the gauge should read 10% higher than when cold. So if you fill to 40psi, the reading should be 44psi. Don't know if this would work for the rear tires on a trike.
Another way, is to check the tread. Ride lots,then check the tires.
If they wear out in the middle, too much preassure. Wear out on the edges, not enough.
Change tires, increase or decrease psi as appropriate, and do it again.
After a few sets of tires, you'll find out what the psi should be for your trike, loading annd riding style.
Of course, if you ride solo, then 2 up, then loaded with 2 up, gear, trailer, etc, the perfect psi will be different for each circumstance. Prepare to ride a lot, take some time and have plenty of tires on hand.