Charging an AC system correctly isn't hard, even without a machine. I have a home-brew setup that works pretty well. It's a bit tedious, but it works well.
HF venturi vacuum pump and gauges, a scale that does .1 ounce increments, and a 30 lb can of R134a.
I bought the 30 pounder because at the time I was building this I had the Expedition with rear air, so it took about 4 lbs, and my cost on a 30 lb can was actually a little less than buying 4 lbs worth of AC Pro.
The thing you have to watch really closely with those DIY jobs at the parts stores is that
1) The have sealer in them. They say it's safe, but so do alcohol companies, and it you look past the bribes and marketing you find that alcohol, like tobacco, is a known carcinogen.
2) R134a cannot be accurately charged with pressure alone. It is too volatile and changes density too quickly with temperature. This means the most accurate way to charge it is into a dry system and weigh it in. Unlike R-12 and the ester oil used up until 1993, R134a does not mix with PAG oil and carry the oil through the system. It pushes the oil around the system as it moves. Since the receiver/dryer is basically a big grease trap in the system and also the last thing the refrigerant goes through before going back to the compressor in a gaseous state, an undercharged R143a system can lead to an oil starved compressor. The guts of your AC compressor look a lot like the guts of your engine, and we all know what happens when those don't get enough oil.
Welcome Visitor! Please take a few seconds and 
