We all have been read or been told to use some magic number like 80% of the old charge when retrofitting to a new refrigerant, but is that going to get you where you need to be? How can one number be valid for all the refrigerants that are now available and those they’re replacing? Sounds like a “Rule of Thumb” to me.
To get a much closer estimate for the required weight of the new refrigerant, you need to compare their liquid density. By using density you can calculate what the equivalent liquid volume of the new refrigerant liquid will be…and that will be a much better guarantee you’ll get it right.
Here is a chart listing various refrigerant densities and a simple formula to use to calculate the new weight needed: