quiksilverly

I found the high and low side service fittings. I have a snap-on cart at work so now my question is this:

When I removed the radiator support during the rebuild, I opened the a/c system and evacuated the pressure. I assume some of the oil is still in the compressor. Is it ok to hook up the cart, turn on the A/C and let the compressor suck up something just shy of say 650kg 134a? I'm afraid the only way to know what's in the system is to suck it down first. Also. Usually on aircraft you service to a certain pressure and then you know you have enough refrigerant. But when you know the total capacity, you can just add the exact amount of 134a and it's good. Is there a pressure switch that keeps the compressor from comming on? Any insite would be great. I wonder, are there any Audi mechanics on this forum? One of them must own an Audi and frequent this place. just curious.