Micron guage is for moisture. Let's you know much moisture is left while pulling your vacuum. Lower the number better off you are. I shoot for below 500 microns. A higher number will be less efficient and parts will where out faster.
I put 6.5 inches down when I put in my Mohawk. I'm 6' 2" and I can lift my Silverado 2500HD and walk under no problem with 12' 6" ceiling height but I have only a few inches to spare to the ceiling.