the original fill capacity of gas was 790~800 grams, the oil was approximately 120 ml (dry fill) - this was spread across the whole system, meaning some in the compressor, some in the condenser and some in the dryer etc.
To flush the hard parts - firstly if your system has baffles in the suction line, that cannot be flushed, you need to replace the line, Use Acetone to flush A/C lines and use compressed air clean the acetone and finally purge with nitrogen if you can.
Replace the expansion valve with a new and genuine part, use a single capillary valve. The twin one is obsolete and no longer available. It basically has a bypass built into it.
make 1000% sure the condenser is good and the O rings are installed with nylog (A/C supply shops sell it).
Do not let your technician charge the system by inverting the cans - liquid can damage the valve plates in the compressor. Let the system suck on it own power, to get the last bit of gas out of the can get a pan of hot water and dunk the refrigerant can in it as it is charging.