I came across this about 2 or 3 years ago. http://www.poolcalculator.com/
Originally thought it was from the Pool Forum or PoolSolutions but think it might be from TFP. Does anyone on the PoolForum use it and is it sufficiently accurate to use? It seems to work fine for me. If not a PS/PF product do you have one available? Supports BBB method of pool care. Thanks.