Or you could go the easy route and get a copy of the centrifugal pump curve, and measure the amps the motor is pulling.
Convert the amps to HP, and along with the delta P across the pump (discharge minus suction pressure) converted to head feet and voila!
Plot it on the curve and you have your flow.
After several measurements you could use as a graph to indicate flow at various discharge pressures, assuming the suction pressure is constant.
