I don't think you can get a reliable micron reading in a mobile a/c system.
dry nitrogen is a very very good way to test for leaks, as long as you have SOME refrigerant in the lines for the sniffer, or you use a ultrasonic tester.
the problem with evacuating a system, then unhooking the pump, and gauges, then hooking gauges back up, then charging is: each time you open the caps, unhook and reconnect gauges, you are putting moisture and noncondensibles in the system. no way around it. you ARE damaging your system.
best to flush out with nitrogen, evacuate again, purge with refrigerant, then evacuate AGAIN, then add in the proper volume of refrigerant.
period.

you NEVER want to leave a mobile a/c system in an evacuated state for longer than the time needed to connect the charging cylinders to the manifold.
period.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!