An electric machine assembly has an electric machine having a stator and a rotor. The rotor has a rotor temperature and is configured to rotate at a rotor speed (ω). The stator has stator windings at a stator winding temperature (tS) and the electric machine defines a number of pole pairs (P). A controller is operatively connected to the electric machine and is configured to receive a torque command (T*). The controller has a processor and tangible, non-transitory memory on which is recorded instructions for executing a method for determining a total permanent magnetic flux (ψT) as a function of the rotor temperature. Execution of the instructions by the processor causes the controller to determine a high-speed magnetic flux factor (ψH) and a low-speed magnetic flux factor (ψL).