To be honest, I can't remember how I originally figured it out. It is a prescription food, I may well have asked the vet.

They all get almost the same amount.....1/4 cup of dry twice a day. The smallest cat gets a skimpy 1/4 cup, the bigger ones a "heaping" 1/4 cup. They do not lose and they do not gain weight, that is how I know it is enough.