I have always thought that when a system is short of gas that is running on R407c or like , that if more than 10% has been lost then rather than "top it up" it should be recovered and recharged because you don't know which of the blends has been lost....but if that is the case then when you ( for example) charge a system with say 3kg from a a full cylinder then how do you know what percentage of each blend is going to be charged into the system...(hope that makes sense)