We have a 3700 gallon vinyl pool. Our TA is at 280 and pH is 7.8. I want to try this method... it makes sense to me. I have two questions ... Our Phosphates are at 500 right now, should I do a PhosFree treatment first? Should I try to keep the Chlorine at the normal swimming level during the process of lowering the TA?? or should I raise it up the Shock level for this treatment?