Go to auto zone and look at the gauge that comes with one of those recharge kits. See what the "green zone" is on the gauge. The pressure is determined by what type of refrigerant the system uses, so it would be the same for any system running R134.
you have two pressures when the a/c is running, the low side and high side. the charging port (near the accumulator) is on the low side of the system. 25-35 psi is good, and listen for the a/c clutch to cycle every few minutes, not every few seconds...
the high side should be 150 ish, higher in a high heat, humidity area.
Your ambient pressure (when the car isn't running, or a/c has been off for a few minutes) is where you get your 1psi to 1 degree comparison, but if you have high humidity, the numbers don't work out that neatly (usually 12 psi for 10 degrees in humid states)