The pressure is a representation of the tempurature, high side for the condenser, low side for the evaporator.
Here is a temp/pressure table:
You want the evaporator as cool as possible without ice (ice starts at 32 degree), therefore, it should be around 30 psi. The condensor should be as cool as possible too, 140 to 160 degrees, therefore, around 210 psi. When the R12 is charging, I use to put a big fan in front of the car to cool the condensor.
Have a nice hot day.