How to determine the reference rate of shearing and rate sensitivity for power-law rate dependent crystal plasticity formulation?

In single crystal plasticity, the material strain hardening is specified by the slip system strain hardness which is determined from the strain hardness function (Eqn 1, see attached). Its parameters can be determined by fitting the equation to the experimental stress-strain curve. In my case, I am determining the parameters by fitting to experimental stress-strain curve obtained at 1000/s strain-rate. However, I am confused on the determination of the reference rate of shearing (a0) and rate sensitivity (n) for power-law rate dependent formulation (Eqn 2, see attached) at 1000/s strain-rate. Almost all of the crystal plasticity literature uses 0.001/s for a0 and 10-20 for n. How do I determine the values for a0 and n at 1000/s strain-rate? Equation 1 Equation 2