It is probably easier to do your calculation at high speed since that is the pump you have now. Use your current filter PSI to calculate your head and figure a 5 hr turnover for high speed and that should be 2 turnovers in 20 hrs at low speed. The Whisperflow 1 HP will do a 5 hr turnover on high speed (assumes 55 feet of head). So I would say either a 3/4 or 1 HP. I would probably go with the 3/4 just to save a bit more on energy use given you will be running it 24/7. BTW there is only a 5 GPM difference between the 3/4 and 1 HP on low speed.