The above is for setting the proper goals, but if you wanted to translate an FC into the weight of chlorine gas which is how SWGs are rated, then use "chlorine gas" in the FC section or in the "Effects of adding chemicals" section. You obviously don't add any of that directly (you set a % ontime instead), but it will tell you that 2 ppm FC in 20,000 gallons is 5.3 ounces weight of chlorine gas. Since most of that FC loss is during the day, one can figure that this amount of chlorine should be generated over 8 hours so that gives a 24-hour rate of (24/8)*5.3 = 15.9 ounces weight or roughly 1 pound chlorine per 24 hours.