I was once told by an older refrigeration guy, that if you block off your condenser coil and let your head pressure get up there (simulating a warm day) and then remove the cover. You should not see any bubbles in your sight glass.
Thinking back to the Theory on how this stuff works, we know that we need a full column of liquid at our TXV and our recievers job is to store liquid. Anytime our pressure drops rapidly by 15 psi or more, our refrigerant will flash, showing bubbles in the sight glass. (this is what we get with a fan cycle control.) So if our pressure drops and we send hot gas to our receiver (which is full of liquid) we should see no bubbles in the glass.
Also we are backing up refrigerant in the condenser and "condensing to a liquid, thus when the valve throttles back, hot gas is then sent to the condenser coil, which is full of liquid) the result, no bubbles.
Not sure if this is a "correct" way of doing it, but It has worked for me and I can't find any reason for it not to. I always weigh in my charge, so I can agree with the other ways, to a point, It's just hard to put a 2# a HP on every system. These older Refrigeration guys have been doing this a long time tho, and usually know their stuff.
Just remeber, When in doubt, go back to the basics