Alright! This is what I do for a living.
First. Your boss must be living in 1972 if he just gave you a set of guages and a few cans of refrigerant to do AC work.
FEDERAL law states that you can't vent any refrigerant. This is their way of making you buy a recovering and recycling machine.
Along with this machine you also need a refrigerant identifier and stop leak detector.
Before you go sucking refrigerant out you must know what it is. If you just blindly go sucking cars/trucks down your going to end up with a contaminated batch in your machine. You don't know what Billybob put in that system before they called you out. You don't want to have to deal with 20 or so lbs of contaminated refrigerant you can't use.
You don't want to **** any stop leak into your machine either. Sucking stop leak into your machine is $1500 every time to repair it. Been there!
You also need a way to pressure test the system. We use Nitrogen and I test them @ 200psi and go over everything with soapy water to try to find a leak.
If no leeks are found I add a few ounces of compatable oil and die. Recharge the system with the machine, run the car for 30 minutes then look for the dye.
When your boss was doing AC work in 1972 you could buy a can of R12 and just shoot it in. R12 systems were very forgiving when dealing with over or undercharges.
R134a systems do not have an oil sump like R12 did so the refrigerant moves the oil much like adding 2 stroke oil to gas in a chainsaw.
Just a few ounces low on charge will burn up the compressor due to lack of lube. R134a has a much different expansion rate than R12 did and a few ounces over charged can explode lines and damage the compressor.
I know some people get away with shooting cans in..Not every body gets shot in Russian Roulette either but eventully they will.
So tell your boss to take back the guages and small can.
At minimum to do it right and legal he needs a recovery and recycling machine, Refrigerant identifier, stop leak identifier, Nitrogen and regulator plus various O-Rings, oils etc.
Plus a way to run the 120v machine out in the field(generator).