Random walk

If you want to calculate dielectric constants from experimental measurements of transmission or reflection, you can use this great software.

RefFit

First step is to fit the spectrum with Drude-Lorentz model and then fill the Kramer-Kronig requirements. Once the software did it for you, just check the consistency of the model with your input. If it fits, just use the results!

I just used it and that works very well.