With R12, on a 75-85* day.... A/C is set to MAX....with the blower on HIGH.... with the temperature setting set to full COLD, the low PSI should be around 32-42. Those pressures depend on the outside ambient temperature. With a fully charged system, the compressor should not be cycling on and off at this time. If the compressor is still cycling on and off, add more freon. Once you see pressures that are relatively steady in that range, check the air temp output of the center vents to see how cold it is. When the blower speed is lowered, the low pressure will start to come down as well as the high pressure. Once the low side pressure gets around 26-28psi on the gauge (depending how the cycling switch is set from the factory), the compressor should start to cycle on and off. When outside temps are in the 90's and higher, the pressures will be higher, and the compressor won't cycle on and off on low blower speeds. Also, expect slightly warmer air temps out the vents when you use NORMAL mode on the A/C panel. When it's set to MAX, your A/C will cool recirculated air from the cabin better than cooling 95* outside air. Of course, all of this is assuming your entire A/C system is mechanically sound.
