I found a Hayward pump/filter sizing .pdf that tells how to calculate which pump and filter you need. Based on that document, with a 21" filter, I need a pump that produces between 12 and 44 GPM @ 30' head. According to your suggestion of 12 GPM/sqft - 27 GPM would be the max flow. What would be the minimum flow required to adequately backwash a 21" sand filter - the Hayward S210T in particular?