1st of all this is an awesome site. Wish I would have had it the last 10 years, but looking forward to better pool care for the next 10. I have a question about the Chlorine/CYA Chart. I understand the realtionship between CYA and the Minium FC and Target FC. I don't understand the Shock FC number and where it fits in the equation or chart. I have just refilled after an 80% drain to correct my CYA. Once I get everything balanced I will be using Borax. My question stems from trying to get my Chlorine right as easily and cheaply as possible without increasing my CYA too quickly. I am using an auto Chlorinator and I average 2 tabs per week. I also shock weekly (In the summer) with a cup on Cal-Hypo 73%. I just want to know more about when and if to shock, or what else I need to do.