Here's how it's done on a production OEM vehicle in a dynamometer climatic wind tunnel. You'll have to take whatever shortcuts, since I doubt you have a dyno wind tunnel.
Start with an evacuated, dry, leak free, everything working, oil in it, ROOM TEMPERATURE system.
Guess a starting charge. Guess way low, like maybe 2/3 of your expected charge level. Use either a heated bottle like a dial-a-charge, or if in the field set the bottle out in the sun to get the refrigerant warmer than the A/C components. Use a scale to measure how much refrigerant mass goes in, and put the guessed initial charge in the system.
Get the wind tunnel up to conditions (Varies by OEM, but generally a very hot high load condition) (Chrysler 110F, 19%RH) (Ford 110F, 50%RH) (GM can't remember, but close to the others)
Run the vehicle to get everything up to normal operating temperatures.
Run the vehicle in recirculation mode, A/C on, full blower, all windows open, driving at (varies with oem) 25mph to 30mph, for 15 minutes to stabilize conditions and operation, record all temperatures and pressures in the refrigeration loop, and air handling systems.
If any "bogey values" are exceeded, immediately stop the test and add 25g more refrigerant (or just add it quickly while running). Typical bogey values are compressor discharge temp>250F, Low side pressure
Add 25g every 15 minutes, recording total charge amount and all refrigeration loop and air handling parameters every ten seconds.
Stop the test when the condenser out sub cool reaches the "target" value (varies by type of system and OEM, but 20F sub cool is typical)
End of test
The challenge is to keep the conditions constant enough so that you can compare each charge level. I suggest you stop charging right after the end of the "flat spot" or "charge window", let me explain.
As you are adding charge you will initially be undercharged, the low side pressure will be too low, the high side pressure will be too low, the vent temps will be too high, compressor will be running too hot.
Then things will get better, and then you will see a "window" during which additional charge really doesn't seem to change anything. This window is the process of filling up the receiver. Changing the level of refrigerant in the receiver has only a barely perceptible change on refrigeration loop operation.
Once the receiver is full, additional refrigerant will be stored in the bottom passes of the condenser. Sub cooling will increase very quickly and high side pressure will increase very quickly (when additional refrigerant increments are added). This is the point at which the system is operating the best, the most efficient and is fully charged.
There are 10 kinds of people in the world, those who understand binary and those who do not.
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