If the pump in question is from an existing manufacturer then the pump displacement should be published. The formula is then basically the displacement x the rpm. However, in am impeller pump, the rpm and back pressure have a dramatic effect on the output of the pump because of the designed internal leakage. At low rpm, a higher percentage of flow will pass by the internal passages. At higher resistive pressures, there will also be greater internal leakage. Therefore the output is not linear with the rpm.
