I need help with sizing a sand filter. Money is an issue, so I would like to get the smallest filter (price) and yet provide adequate filtering for the pool. I have a good pump, but don't know the flow. The pool store says they need to know the flow before they can recommend a filter. It is a 1HP pump made by Hercules, w/ 1.5" in/out. It has no flow markings at all. I don't want to undersize the filter flow and burn out the pump. The only info on the pump is the motor info itself, which is a Century motor. Your assistance is appreciated.