for shops that do not have a refrigerant charging scale, one inexpensive option is to use robinaire's dial-a-charge charging cylinder. it will show how much refrigerant is being charged in relation to the liquid in the sight glass (its equivalent in weight) with ambient temperature compensation. its usage eliminates the guesswork out of charging using the manifold gauges readings as a basis. with it, you can also use and must use the pressure/temperature chart to be able to properly interpret the manifold gauge readings.