I am trying to figure out my GPM for my older inground spa. I cant seem to find a specific formula for spas only pools with a min. Of 10,000 gallons. What turnover time do i divide my gallons by? I have a 690 gallon inground spa, 4 jets,1hp whisperflo, 1 skimmer, 1 drain, 1.5" piping, 3 90 deg. elbows and 4 on tbe return side. The heater is a 150k btu minimax and the skimmer is 22' away from pump. I am trying to get my gpm to be able to select approiate filter (cartridge).