I don't know what would be the amount of overcharge in ounces. The problem with overcharge is excessive high side pressure, so that is what should be watched.

As far as overcharging it would be worth mentioning here that overcharge is not too much of a problem in an accumulator system. It is however a problem in our benzes with a filter drier system.

When charging by sightglass, you should charge the system a little at a time with engine speed at 2,000 and preferably a fan in front of the car to simulate ram air. Charge it a little at a time until you almost see a clear sight glass with an occasional bubble. This is how you keep from overcharge. As leathermang says its better to be undercharged than overcharged.

BTW, in a converted system, the sightglass means nothing. It is only useful if the original refrigerant that the system was designed for is being used.

Good luck,