R134 VA system charging,

[badazz81z28]

So I’m curious. You see so many off the shelf charging systems that come with gauges to charge “until your in the green” and others like VA give you the weight. I’m looking at R134 cans and some are 20oz but the back of the bottle says 18oz R134 and 2oz of additives. Do I just go off the R134 # or total of oz? Also why can’t i use the home systems and charge until the gauge shows green? Seems a lot easier than trying to measure by weight which I think would be less accurate. I have my system sealed and Evac all ready to go. Just needs Freon.