All pressures are temperature dependant. At the very minimum, an R12 system should show about 60 psi just to keep the system from freezing up. That's with an ambient temperature of 70 - 74 degrees farenhiet. As the ambient temp goes up, so should the low side and high side pressures. If you don't know a lot about air conditioning system, shoot for a low side pressure of about 70 psi at 80 degrees ambient. Invest in a pocket thermometer and check the ambient temp and the air temp coming out of the center A/C vents on your vehicle. Outlet air temp in the 40 degree range is optimum. By the way, a good thermometer is just as important as a gauge set when servicing the system. To accurately check ambient temp, hold the thermometer in front of the grill so you're measuring the actual air temp going through your condensor.
Tom