Dave I am not sure I can answer this question as well as you might like me to but let me try.
Let's start with the two extremes. A) Not enough refrigerant, and B) too much refrigerant. If the refrigerant is low, the clutch will cycle out (e.g. turn the compressor off), and the refrigerant at the evaporator will expand and warm up before it is fully through the evaporator, leading to inadequate cooling. If the refrigerant charge is too high, the refrigerant will not have fully expanded by the time it gets across the evaporator, wasting cooling capacity. (This can also be bad for the compressor as it might be trying to compress liquid - sometimes called a flooded evaparator). The ideal is somewhere in between.
Put another more technical way - the inlet and outlet of the evaporator should be between 5 to 10 degrees or so apart (representing the superheat of the system... superheat is a concept based on physics and the latent heat of vaporization and latent heat of condensation - not super complicated concept yet not worthy of the time and space here).
Ok, that out of the way, here is what I could recommend where you have a system needing refrigerant - with enough to run the system yet not adequately cool it... Run the engine and start the AC. I'd prefer cabin temps be 70-75 degrees, and I'd have you put a fan in front of the condensor to aid in heat removal. Maybe even squirt the condensor with a hose a few times to help get things stabilized). Monitor your vent temps - increase the engine rpm to 1500 and keep it there for the duration of this procedure. After 5-10 minutes you can start. Add refrigerant until your vent temps head down... slow down a bit and continue to add about 3-4 oz at a time until the vent temps fail to go any lower. Then ...Add another 2-3 oz and see if the temps increase, if not, call it good. If so, bleed that back off and leave it as is.
It is a little easier to do this with equipment that lets you weigh the jug as you go (to 1/4 of an oz.) and will also recover excess charge.
Alternatively, to be able to recover the full charge and separate the oil is heaven, because then you can put in the exact factory charge and know where you are on oil as well... but then... that means taking it to a shop as a general rule.
Anyway, this is not an exact science, but that's my answer. Hope that helps. Stabilize the lowest vent temps is the short answer.