When an automotive ac system has to be opened for repair the refrigerant (was R12, since 1995 it's R134) is recovered and later reinstalled. Following repair the system is vacuumed to 29.9" mercury (I think that's about 500 microns to you guys). If the system holds vacuum it is recharged by weight to a precise spec. Most systems nowadays hold 1 to 2 lbs. The high and low side pressures are only checked to ensure the mechanical components are functioning properly. As a matter of course the amount of refrigerant is never adjusted up or down.
I understand that you guys look at superheat, supercool, and pressures and adjust refrigerant accordingly. Is this because there are many variables we don't deal with in the automotive ac biz? Like line length, mismatched condensers and air handlers? Or what?
Also, do you recover the refrigerant when you open a system? I understand some or all of it can be held in the condenser. How do you get it there? Do you seat the valve on one side and pump down the lines and the evaporator and pump it into the condenser?
(As an aside, if you think you've dealt with some hackers in your trade you wouldn't believe what we see in the automotive trade. How about filling the system with propane just for starters? Our Wall of Shame would have you figuring out whether you should be laughing or crying!)