in order to get a real reading on the vaccum you need a micrometer, the cheep ones are junk and you will pay about $500 at the very least for a GOOD one that will actually work. Rule of thumb is, vacuum it down for 1 hour and turn off all the valves and let it sit overnight or for a full day. if the needle moves AT ALL, you have a leak. To truly get down to a level you should be at for a perfect job, you need to flush all your lines out and change the oil in the compressor and new dryer because of contaminates. In a nut shell, just vac it down for 1-2 hours and let it sit for 1 full day. if nothing changes than your golden. if the system always had a good charge on it than just weigh in new refrigerant, dont do this any other way. If its in need of service, change all your o rings, the valve, dryer, and flush all the lines and evaporator with AC flush followed with nitrogen purge. Pull a vacuum for 1 hour and let it rest, if good, fill with nitrogen and do a bubble test with soap water than vacuum it again. Now weigh it back in and enjoy. there is more to meet the eye about this stuff and if you want you can PM me and i will help you out. My father is an Master HVAC and know too much about this for my own good.