If you are one of the guilty folks who slam in a can of AC charge to "top up", this video from FordTechMakuloco goes over how to figure out you went too far and overcharged the system. Enjoy.
How to recognize an overcharged AC system
- Thread starter Snagglefoot
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This is a lot like a oil analysis. If you don't have a baseline to go by then it's just data. If you don't have a known accurate pressure to compare against then you can't just go out and hang a set of gauges on a system and know whether it's overcharged by a lb. of refrigerant or not.
I have tried to use those temperature/humidity charts to compare against pressure gauge readings and that information is also all over the place. It seems to vary by system and moon phase.
Bottom line, evacuate the system and recharge to a precise and accurate quantity and let it go. Anything else is simply guessing. If you have no A/C and it's 95ºF outside then any sane human would drop $10 on a 12 oz. can of R134a and see if they can improve their situation.
I have tried to use those temperature/humidity charts to compare against pressure gauge readings and that information is also all over the place. It seems to vary by system and moon phase.
Bottom line, evacuate the system and recharge to a precise and accurate quantity and let it go. Anything else is simply guessing. If you have no A/C and it's 95ºF outside then any sane human would drop $10 on a 12 oz. can of R134a and see if they can improve their situation.
Although if you are too low and compressor will not kick in, then you might not get the R134a to flow from the can into the AC system.
My view is unless you have all the tools like an A/C service machine you should bring it to a shop. If your A/C system lis not working, then you probably have a leak (or leaks) that needs to be found and repaired then hook up the A/C service machine.
Oil changes are easy, properly servicing an A/C system is a lot more complicated.
The only proper way is to evacuate and charge by weight. Variable compressors that are now pretty common are impossible to charge by pressures only. It must be weighed in.
In my experience using the old R12 back in the day, no gauges, dirt cheap. This was the way to tell : System obviously low, compressor cycling and blowing 50 degree air from the vents using a stick thermometer. Bypass the compressor to make it run, dump in a 1lb can, right side up. Let it run in the heat 95 degrees, high humidity, at 1200 rpm. Check thermometer. Oh, it got colder by 8 degrees. Dump in another can, oh, almost 33 degrees ! Dump in another can, thermometer went up and high line stopped sweating. OOPSY !!! Overfilled. Disconnect and push schrader valve in to let some out. Keep checkin thermostat till you got to around 33-35 degrees, YAY 
It's full. Then throw in a can of compressor oil for the hell of it ....Good ole days
It's full. Then throw in a can of compressor oil for the hell of it ....Good ole days
I think the above was in the Ford service manual for a Pinto. And shame dumping R12 into atmosphere.In my experience using the old R12 back in the day, no gauges, dirt cheap. This was the way to tell : System obviously low, compressor cycling and blowing 50 degree air from the vents using a stick thermometer. Bypass the compressor to make it run, dump in a 1lb can, right side up. Let it run in the heat 95 degrees, high humidity, at 1200 rpm. Check thermometer. Oh, it got colder by 8 degrees. Dump in another can, oh, almost 33 degrees ! Dump in another can, thermometer went up and high line stopped sweating. OOPSY !!! Overfilled. Disconnect and push schrader valve in to let some out. Keep checkin thermostat till you got to around 33-35 degrees, YAY
But for a decent car you need to find & fix the leaks. Pull a vacuum for 30 min to check for leaks, then fill by weight.
I used to work on systems that didn't come with data like charge weights. So when charging I went by pressures.
