yeah, that's right. But besides that, I was referring to (see below), once being in PBR, the assumption of insulators with specular F0=4%. I think you've addopted the best choice in this matter.
Now, most important than that, i've to understand all this
I quote: http://www.marmoset.co/toolbag/learn/pbr-conversion
" [...] The biggest difference between the two workflows is how the diffuse and reflectivity content is defined in the texture maps. In the specular workflow, these values are set explicitly with two unique texture maps.
The metalness workflow on the other hand, uses the albedo map to define both the diffuse and reflectivity content and the metalness map to define whether the material is an insulator or a metal. "