Kinematic Harderning

Hi all

I am currently investigating Prager hardening room which is stated as the simplist linear kinematic hardening rule. I was wondering if anyone has worked with this model and if so if you have managed to identify how to determine the constant used for the back stress equation

dα =C * dεp ( this is epsilon dot p which is the equivalent plastic strain)

Please can you let me know how to determine the constant C from experimental results?