hmm. ok. so how do you know how much is in the system / how much to add/how not to overfill?
The manual will tell you how much oil to add when you replace a specific component - compressor change-out gets X oz of oil, evaporator coil or accumulator-drier gets Y oz of oil. Or, more accurately - the new component needs to have those amounts of oil in them for the replacement.
if you have a leak and take care of it within a reasonable amount of time - No problem, don't worry about adding any more than the leaking component calls for.
If you have a leak and are just constantly adding refrigerant (the stuff without oil added to it) rather than solving the actual problem - then you really can't know how much you actually need to add without removing components and measuring the amount of oil in each one - which would not be fun.
There is one other way, it's still a pain in the ass, but it's less of a pain than removing ALL the components to measure the oil in each one. Once you get the system leak free - fill it up with refrigerant and run it for a good long while - take a drive on the highway to get it really going, and turn the fan onto the highest setting. Then - recover the charge, remove the compressor and measure the amount of oil inside of it. If it's low - fill it up to the manual's specifications. If it's too much - remove some.
The compressor is the only thing that needs the oil - the rest of the oil in the system is there simply because there are places where it falls out of suspension with the refrigerant - so if your system is fully charged and has run for a little while the oil will naturally just accumulate in those spots other than the compressor, and those levels will remain pretty damn consistent so long as everything is working properly. So really after the oil settles out in the rest of the system, all you have to worry about is what's in the compressor.
So in short - you can't know how much oil to add without doing some kind of invasive procedures.