Any decent A/C tech should be able to determine the charge rate using gauge readings and, more importantly, temp readings on the input and output of the evaporator. Its all science, but not an exact one. Once they determine the charge rate, you can document it for future use.
The PAG oil issue isn't so much a mystery if you follow the FSM for the system the compressor was originally used on. Assuming you are using all new components, which ideally you should, the amount of oil indicated in the FSM should cover you. If the compressor isn't new, you should drain as much oil as you can from it and refill it according to the FSM. If I remember correctly, the general rule for other components is 2-3oz of PAG for each new condenser, evaporator, and receiver/drier. New compressors usually come with instructions on how to calculate the oil charge. They have you drill a hole in the bottom of the old reciever/drier, measure the oil that drains out, and do some basic math to calculate the amount of oil to add. Overall, its generally better to have a bit too much oil rather than not enough. VERY IMPORTANT: Different compressors use different weight PAG oil. A bottle of OEM spec oil is way cheaper than a new compressor. PAG100 is not universal!
If your components are from a system that has been disconnected or has had a compressor failure and new parts aren't an option, it is wise to thoroughly flush each component out with solvent based a/c flush (or brakleen) and massive amounts of compressed air. Any orifice tubes, expansion valves, mufflers, receiver driers, or accumulators should be replaced.
A bit overkill, I know, but hopefully that about covers it for others as well. That's all from my experience dealing with customized A/C setups in limos and shuttle buses.