A/C Pressure

It depends on the refrigerant you are using (12/134/something else). Also, it is not an accurate way to determine the charge BUT...

With R12 the statlc pressure should be approximately equal in PSI to the temp in F. So if it's 80 degrees, the static pressure in the system (with the car just sitting there, preferably after sitting overnight) with R12 should be about 80PSI.

The operating pressure will also vary considerably depending on the ambient temp but 28-40PSI is the usual low side range with the high side anywhere from around 180 to 250, again depending on the refrigerant and the ambient temp.

With that sort of variation, you can see that pressure alone cannot tell you if the system has the proper amount of refrigerant. Weighing the refrigerant on install is the way it is done at shops. You can get even more accurate if you are willing to fool around with slowly adding refrigerant to get the optimum difference between ambient air and evaporator vent air and then, when the difference starts to narrow again as the system gets too much refrigerant, bleed the refrigerant off back to that optimum difference point.