It was created by Jason Lion at Trouble Free Pool (TFP) using formulas and table approximations he created from my spreadsheet. There is no PS/PF calculator and it is fine to use for calculating dosages. Just note that the calculations regarding the amount of acid/base needed to adjust pH are approximate. Also note that the recommended FC levels don't exactly match Ben's Best Guess Chlorine Chart as they are instead based on levels of consistent active chlorine (hypochlorous acid) I calculated, though the baseline for this was still based on the active chlorine levels at the mid-range of CYA in Ben's chart.