For the case of a non-uniform plasma density, one can use the
characteristic matrix method of stratified media (Born and Wolf
"Principles of Optics" Chapter 13.4) for absorbing media in conjunction
with Drude model to calculate the average plasma density at the
wavelength in question for a desired reflectivity (assuming the plasma
density as a function of depth can also be modeled). As another
reference, see T.E. Wilson, "Modeling the high-speed switching of
far-infrared radiation by photoionization in a semiconductor", Phys.
Rev. B, 59, 12996 (1999). For a back of the envelope calculation, B&W
gives a simple formula for both very thin and very thick (compared to
the optical wavelength) conducting films.

Thomas Wilson
Marshall University