Hi!

There are a numerous techniques for evaluation of the rotor flux reference. It depends on equivalent circuit to apply, power losses and non-linear characteristics to take into account, etc. From my point of view, if one use the vector control with rotor flux oriented scheme by aligning the reference frame on the rotor flux axes then equations from the attached could be utilized (I used this equation for simulation in Matlab some time ago). This rated value of the rotor flux reference was yielded from theoretical background based on T-shape equivalent circuit without any iron losses, non-linearity. For more complication it needs to take the features mentioned above into account. It was done in abstract (see link), the equations (32), (59)-(61) and fig.4 show the way the rotor flux reference might be expressed.  

http://faculty.mu.edu.sa/public/uploads/1338145476.8684nam.pdf

Thank you Alexander  for your kind help