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This won't help much while doing a backyard fill, but it is interesting information nonetheless.
A buddy of mine used to own/operate an independent shop with a nice Snap-on A/C evac & recharge machine. The digital kind that you just punch in a requested freon weight (once it held a vacuum to your satisfaction) and it recharges to that specific weight then shuts off. While charging different systems, we noticed that most cars got really super cold at the vent when they were almost done charging. Once they were at the specified amount however, they seemed to have lost that edge. My particular car, a Datsun ZX, listed 2.0 pounds as a standard capacity on the sticker. It never did cool well, even when new. But it did cool some, just not enough and definitely not great. But I can say it functioned with 2.0 pounds in it.
We had sent my compressor out to be rebuilt (for a small compressor shaft seal leakage) and had it back in my car with a new dryer and did the full evac and charge to 2.0 lbs and it worked, but did not even cool quite as well as before. We started playing around with different undercharge amounts on this car by percentage and it turned out it cooled better then ever before at 1.84 lbs or 92% of the listed standard charge.
If you ever get access to a system that makes it that easy to fill to an exact amount of refrigerant, try that out as a baseline tweak and I would be surprised if it didn't work. The best we could figure out was that some folks add the prescribed quantity of PAG oil in addition to the freon total and the system is effectively overcharged by the additional volume. But who knows?
I had that car for another ten years and it set a new standard, consistently cooling very effectively. Before that it totally sucked for the eleven prior Texas summers starting from a brand new car. We just knew the thing consistently blew ice cubes at me from then on.
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