well the refregrant weight is written on a lable pasted on condencer fan and also on the eveporator casing ,,,,,,,,,,,,,,you cant weight the actual gas left/pressent inside the system,,,,,,,,,,,to check the gas left inside or to check wether its fully charged or not you need manifold guage set,,,,,,,,,,match the low and high side pressure readings with the corresponding reading of amibent temperature chart,,,,,,,,,,the readings will differ as the ambient temp changes,,,,,,,,,,,and keep in mind you can judge any thing with the static pressure,,,,,,,,,,i have seen a guy on ac shop asking same question as you,,,,,,,,the mechnic connect a pressure guage with compressor off and says its low charged ,,,,,,,,,,,,,,,you can judge much from static pressure it also varries as ambient temp increases,,,,,,,,,,,,,,,,,,,,
i guess in your case you feel that your system is running with low refregrant,,,,,,,,,,may be its fully charged and problem is choked dryer or blocked expansion valve,,,,,,,,,also check the cabin filter if its equiped,,,,,,,,,
