The equation I use (as a guide) is ounces of R12 divided by 1.65. So in this case, 3.0 lbs is 48 oz. so divide 48 by 1.65 = 29.0 oz. Or roughly 60 to 70% of the R12 charge. I found the 1.65 method to work pretty well. I monitor the guages and adjust according ... Login to read more.