My Autokool machine actually weighs the old refrigerant as it pulls it out of the system. I've seen cars with as little as 50g of refrigerant still blowing cold air when stationary. The problem is once moving, when the system has to work against the warm air being pushed into the cabin, it cannot cope due to the lack of refrigerant. E85's "should" contain 740g of R134a if my memory serves me correctly. The refrigerant is also hygroscopic, and therefore absorbs moisture into the system, again through the same seal on the compressor, if left in there it reduces the system's efficiency, as liquid cannot be compressed, but can also cause internal corrosion. That's why a "proper" aircon recharge includes a vacuum cycle, it's not just to test for leaks, it does the more important job of boiling off the water in the system. Yes, I did say boiling, as under 29.74" of vacuum, water boils at zero degrees... The vacuum in the system then actually helps pull in the lubricating oil and the refrigerant when charging.
This is why I never recommend the "top up" cans that Halfords and the like sell. No vacuum test, no idea of the weight of refrigerant in the system, and no lubricating oil for the compressor!
And before any says, "so why doesn't my fridge at home need topping up?" the answer is, that on a fridge, the motor and compressor are hermetically sealed in a case, therefore there is nowhere for the refrigerant to escape to!
Mike