Bill,
Most good AC shops deal with this question all the time and should have some method of handling it. Many will pull the R134A with a vacuum and use a weighted charge that will be put back in. The pressure is temp dependent so this is an approximation.
A good High Side Pressure Reading for 134a = twice the ambient temperature + 50.
For Example at 85° F, you would expect a high side pressure of 85x2=170+50 = 220 psig.
The R134 high side value is obtained for the outside ambient temperature that you are charging at. These values to be correct requires that the engine is running at high idle, say 1000RPM, the dash air conditioner fan is running on high, and that there is good air flow over the condenser with the condenser electric fans running. Low side will probably run 30 to 35 lbs.
Later Ed
