I read that you should subtract 1/2 of your CYA measured value from your measured alkalinity. The resulting number is your effective alkalinity value. Is this true? If so does the TFP alkalinity range take this into account?
My understanding of this calculation is for Carbonate Alkalinity . I have found you can correct alkalinity by taking 1/3 of the CYA level and subtracting it from the TA level. To be exact, you will need to know the pH of the water, but since pools are generally between 7.2 and 7.8 pH, using 1/3 of CYA value is suitable. You will see this noted on some Pool Store water test results as they denote Alkalinity is adjusted based on CYA level. But truthfully, the times I have asked them what that meant, no one could really give a suitable answer. Par for pool store expertise!
I found the following on Taylor Technologies website regarding CYA and Alkalinity.
The cyanuric acid/cyanurate system contributes to total alkalinity since total alkalinity is the sum of all titratable alkaline substances and cyanurate is a titratable alkaline substance. Therefore, the total alkalinity titration measures both carbonate and cyanurate alkalinities. This affects water balance calculations because the alkalinity term in the Saturation Index equation is strictly carbonate alkalinity.
As indicated by @mknauss this is already accounted for in Pool Math.