I don't do residential A/C, I'm not licensed or trained professionally. I have worked on automotive A/C, I only monitor vacuum down to 30 inches Hg. Don't deal with micron levels.
Why are you asking? If you're not trained, you shouldn't handle it; if you are, you should know the answers to your questions.
In any air conditioning system / refrigeration system whether it be automobile, home, industrial, or commercial the desired goal is to remove as much air, moisture and other non-condensables from the system. You attain a vacuum to this level and then charge refrigerant into the system. From study that I have made 500 microns is desired level of vacuum that the EPA (environmental protection agency) specifies in refrigerant technician certification.
Look at this chart. The difference between 100, 500, and 1000 microns is very minute. You would NOT be able to detect this difference by a visual inspection of the suction side gauge on a manifold gauge set. So you need a precise, electronic gauge that measures in microns to be able to detect this level.
If you had to open the system for service of some kind, it is also important to know how much oil is left in the system.
Did any leak out or was any left inside a replaced component? Is any debris, metallic or otherwise in the residual oil if any component seized or came apart?
Guessing is bad. Over-oiling can sometimes be as destructive as under-oiling. A measurement of system oil and the procedure for that is usually given in the service instructions.
I agree that an oil charge is entered into the system under a held near-perfect vacuum, followed by a refrigerant charge. If a vacuum doesn't hold over a few minutes, then there is likely still a leak. Don't charge a leaking system.
I always want to see vacuum hold overnight. A few minutes will not show small leaks. Residential split systems have to be charged by pressure s the length of the lines is variable and you won't have a weight to charge by. As noted above, if you're asking this question you need to do some more study to learn what you're up to.
You also need the superheat chart for the refrigerant used and a good digital thermometer for the suction side. I have done both residential and mobile systems. I have a huge magnetic base drill and a power hacksaw from helping a former co-worker get his R22 system in his house straight so he could sell it when he retired.
FWIW, I do have a vacuum pump that will reach the level EPA wants, just don't have the high accuracy (probably electronic) gauge needed. It is a big old Welch Duo Seal pump. It will pull a residential central air system down on the standard gauge set to 29+ inches in about 30 mins. and a vehicle system, even a dual evaporator in about 5 mins. The only problem I have had, it is so strong and has enough capacity it can hide a leak.