Troubleshooting help needed-AC pressures

I went to only using the high side valves for the vacuum... After I vacuum the system down I generally close off both high side valves the one on the gauges and the one down on the high side valve on the unit itself. I actually take it off at that point. Then after it holds 30 negative pounds for a few hours I go ahead start the car turn the AC on high and start charging the system. And definitely when I open the can I purge the lines...
I'm not saying this is "the" way to do it but it's the way I do it...