I've got 16-18 PSI in the rear, and 22 PSI in the front.
I've had one or both front tires go flat three times this spring and I think it was due to my turning the front wheels while not moving and the tire 'burped' off the bead and let out air. The first time it happened I thought I might have run over a nail, but there were no punctures. Once I seated the tire on rim it held air just fine for several weeks. So far it hasn't happened again in the past month while I've put 10 hours on the tractor.