The 20' is a red herring. The 10.5' and the 4' are the pump set points. So volume to be pumped is 3.14x(15'x15')x6.5. If we have 380gpm being pumped out and 96 being pumped in then the effective emptying rate is 284gpm. So V in gallons/284 gives you the total minutes to pump down. You'd need to calculate the rest I'm afraid. I live in the 21st century and use metric.