See the "Chlorine/CYA Relationship" section in the thread Certified Pool Operator (CPO) training -- What is not taught for more detailed info. Even though this chlorine/CYA relationship was definitively determined in 1974 and known before then (in the 60's), virtually no one in the industry talks about it with any specificity. This is primarily due to "the big lie" from the chlorinated cyanurate manufacturers (makers/sellers of Trichlor and Dichlor) that "CYA doesn't matter; only FC matters".