AC guinea pig...

I'm sure you already know this but just for the people reading this. If you do buy one of those pumps that comes with a little pint of oil and you need to put about three-quarters of that into the pump and there's a sight window on the front of it.
With that said you hook up your gauges to the high side (red) the low side (blue)o the AC and hook the pump up to the center yellow line and open the system up and pull from both sides for about 15 minutes. The low side gauge should read about 29 pounds or so of vacuum. at that point close everything shut it off and hope to holy heck it holds that 29 pounds for about an hour or so. And then if it does continue to draw with the pump for another hour at least. This draws all the moisture from the system. Then simply hook a can to that yellow line open it up and bleed the line and then start feeling the system on the low side with the engine running and warmed up. And also have the system turned on and when the system feels it has enough Freon it will turn the pump on and start drawing more Freon into the system. Then it's just a matter of reading your ambient charts...