refrigerant charge formula for r123

I think you're asking about the leakage rate that is used in the EAc5 calculation. If that is correct then the data points that are used in the Credit Form calculator are (1) total pounds of refrigerant added and (2) days since refrigerant was last added. If the service provider does not track those two data points and uses another methodology like you've described above, then you should submit a narrative along with the credit submission demonstrating that their methodology provides the actual leakage rate over the performance period. This is very important because the credit cannot be earned unless the building team has monitored the actual leakage rates. If the actual leakage rate is less than 0.5%, you should still enter 0.5% in the credit form as that is the minimum that the review team will accept.