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)