Fill factor Caculation

I'm modelling a metamaetrial slab on CST and get S-parameters from the results. Then I need to extract permeability and permittivity from the those parameters. I learnt a method based on the Lorentz Model, as
mu(w)=1+(F*w^2)/(w0^2-w^2)+jrw
Note there is a F called fill factor, but I don't know how to calculate in my model. Since I do not use SPP, thus an equation I fond is not helpful. Is there anyone who know how to do it based on a model geometric data?