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DJ,
I am under the impression that the only true way to get the proper amount of Freon in the system is to start over. That is, go through the pulling a vacuum procedure, and then add the Freon by weight. There should be a spec for your car, which will have a + / - tolerance. Because newer systems tend to use less Freon, the + / - tolerance is less, which makes a good guess harder.
Below is a quote from an ac certification study guide.
Charley
Charging Systems with Small Cans or by Attempting
to Use Pressure Readings
Part of this one’s already been mentioned, but If you’re
charging with small cans, or even using 30 pound cylinders,
but trying to charge a system going by gauge readings, or
using any other method that does not involve accurately
weighing the amount of refrigerant entering the system,
STOP!
Pressure gauges are a diagnostic tool, but you can’t use
them to meter refrigerant into a system, and no vehicle or
system manufacturer provides exact pressure specs for a
precise charge. Even if you have reliable pressure readings,
as we’ve already pointed out, there isn’t enough of a pressure
difference to indicate over or undercharge.
System pressure readings, whether the system is operating
or off, will not identify the amount of refrigerant in a
system. The only way to assure that any given system’s refrigerant
charge is correct is to recover all of the existing
refrigerant, perform a thorough evacuation and install a
known amount. If you have a recovery/recycling only machine,
consider purchasing an accurate digital scale, perhaps
one with a solenoid-type dispensing valve.
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