How to model absorbing material in ADS Momentum (reflectivity of 20 dB or more)?

In ADS, one can define a dielectric material with ε*=ε'+jε'' and μ*=μ'+jμ'' with loss tangent being tan D = ε''/ε'. However, ε'' = conductivity/angular frequency, so the higher tan D, the higher conductivity is? How does it relate with reflectivity?

I looked at the reflective materials and their μ* and ε* and I cannot determine how they relate to attenuation in dB/cm (well, they did provide the formula but some variables were not defined) and eventually in reflectivity.

Thanks

http://www.eccosorb.com/Collateral/D...lish-US/MF.pdf
http://www.eccosorb.com/Collateral/D...0Materials.pdf