What determines the "So Called" Capacity or recommended Charge volume. I know the General rule is to use 85% the charge volume of R12 (2.25 lb) when doing the 134a conversion that's (1.91 lb). What determines the Charge Volume of any given system? Is it strictly pressure or a combination or system volume too.

Here is another question. When you see the quick charge 134a cans with a built in gauge, How do you read it? (system running or static off). What I mean, is there a point where it should show a steady pressure in the Blue zone. When our system is running, the Pressure is cycling up and down via the pressure switches in a (fixed orifice cycling type) Does is it read differently an Expansion valve type system? When you are doing a top off and you don't know how much is left in the system, how do determine that right amount? Short of it's blowing COLD again so I guess its good.

Sorry if this has gone off topic.