"Specularity" to "eta"

[sldstt]

Hi, I have a model which contains a "specularity" and "roughness" map.

I would like to use the subsurface material, which excepts "eta" (index of refraction).

How can I convert "specularity" to "eta" in a meaningful way? 

I thought about solving the Fresnel equation but the results I get don't seem to make sense.

Thanks!