For general reference, I don't think 30 minutes of vacuum is enough. What you are doing is boiling off any water that may be in the system and some of it may be disolved in the PAG oil or should I say emulsified? Anyhow, you have to give it enough time to boil off the water into vapor that can be sucked out of the system.
I just did mine to cure freezing symptoms. I was lazy last summer when I swapped engines and didn't evac the system. On all systems I pump it down for an hour and if the rig can be left overnight I leave the gauges hooked up (disconnect the pump and close the valves) and check it in the morning. Any water boiling off overnight will reduce the vacuum toward zero (as will leaks, but that's another story). This time I found it lost a few microns so I pumped it down again for 30 min and called it good.
If a system is overcharged the compressor will likely be noisy as it's getting some unevaporated liquid on the low side. Enough of that will destroy a compressor.
If you don't know how much refrigerant to use you can figure it out by measuring superheat or subcool (depending on whether it's an orfice tube or TXV). Read up on the subject and you'll find how much of either you should have. A manifold set and a digital thermometer are mandatory. This is how techs doing whole-house, or custom, commercial systems figure it out as there's now way in advance of knowing how much refrigerant to use. Robinaire and others have on-line courses on the subject.
