Up until 1990 that technique worked if everything matched etc. The newer higher SEER units run lower suction and discharge pressures and temp split because of higher capacity fans and now we have to use superheat and subcooling. A few old timers like myself have real good instincts. You have to have 10 or more years experience and understand the matchup of the system components, house insulation etc etc. Only a lot of experience gets you to that point.
Oh, yuri, I have heard about the beer can cold method of charging but I havent been explained in detail. I know thats how my boss does it. He charges the system until the suction line is cold and checks for the 20 degree split and goes from there.
If the system has a TXV then that method could be masking the symptoms of an incorrectly charged system.