I have a pool that holds 9500 gallons of water. According to this article http://www.inyopools.com/HowToPage/how_to_size_a_pool_pump_for_your_in_ground_pool_.aspx, based on 8 hours turnover and 50' of head, I need a pump that has 20 gpm flow. That seems to me very low, but if correct that would...