You know, there's some smart guys on here but with a/c it will tell you when it needs more. If the system is healthy and the duct temp is low, add a little more freon, let it stabilize and monitor the duct temp. Take it for a drive if need be with the thermometer in the duct. AC to me is a lot of common sense. If the temp goes up sitting at idle or at a stop sign, check the fan and condenser and radiator for restrictions. If the compressor is noisy, it needs oil or replacement. If a line is cold and all of a sudden warm (and a expansion valve isn't inline) there is a restriction. I have charged dozens of systems without any gauges and just a thermometer.