MATLAB provides lots of ways for you to determine partial derivatives from the flux. One way, for example, is to fit a polynomial and then differentiate the polynomial terms to get the partial derivative. You could also just fit the data (suggest the 'makima' option) and then numerically determine the gradient at the break points. For an example that uses the polynomial approach see https://www.mathworks.com/help/sps/ug/torque-motor-parameterization.html - see Calculate and plot flux data (see code) link from the model.
Hope that helps
Rick
