A control system for controlling operation of a fluid delivery system that includes a first and second pumps for delivering first and second fluids for mixing. A controller includes a first pump map having a first pump flow rate mapped against a control output signal, and a second pump map having a second pump flow rate mapped against a control output signal. The controller determines the control output signal for the first pump to obtain a desired flow rate from the first pump, and the control output signal for the second pump to obtain a target flow rate and a target percentage of the second fluid relative to the first fluid or a target percentage of the second fluid relative to an overall fluid flow. The output signals for the first and second pumps are determined using the first and second pump maps.