I get two pressure readings while checking my low-side pressure, which one should I use?

When checking low-side pressure you will get two different pressure readings, one when the compressor cycles on and one when the compressor cycles off.  Use the pressure reading when the compressor is cycled on and the system is operating at its highest settings (temp to max cold and fan on high). When the compressor cycles on you should see the needle on your pressure gauge begin to drop. When the compressor turns off you will see the needle move back to a higher pressure. Take the reading that occurs when the pressure is at its lowest point when the compressor is cycled on. To tell if your compressor is cycling properly, view the video below: