I have valves at the machine on mine and when ever I charge I charge through the low side valve (the high side valve is closed on the machine) and a "charge" valve that I throttle to control refrigerant going into the car (the valves at the ends of the hoses remain open the whole time). You should have enough refrigerant in the car to satisfy the low pressure switch so if you did not get to your target charge amount then you can run the car with the a/c on (the compressor should run at this point) and the low side pressure should come down allowing you to throttle the remaining amount of regrigerant into the car.

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Jim