One very large chain uses (as of 2000) shop air instead of nitrogen to pressurize the system. No kidding.
Also, it is a common practice to observe the low side needle at 29.9" hg and let is sit there with the pump running for 30 mins. No micron gauge. I'll bet only one out of 5000 auto ac shops even owns a micron gauge.
Two hours and put the charge in. Obviously, you enjoy learning, and in some ways, you are already equaling what you would be paying for, as AC is not a "specialty" in the auto world.
When your charge is in and the system is running, you can spray a soapy solution on those hoses and connections, and bubbles in the sprayed areas will show you all leaks except for the evap, so that's a "process of elimination" for that component.