How am I supposed to use the PBR reference charts?

I've always made PBR materials in a really simple way- I'll paint/photogrammetry/whatever the diffuse, bake out the metallic/normal/etc maps, cram them together in the mat, and call it a day. Looking at https://blogs.unity3d.com/2015/02/18/working-with-physically-based-shading-a-practical-approach/ however, they provide specular and metallic value charts that for the life of me I can't decipher- are those just supposed to serve as general guidelines when making textures, or is there a specific, quantifiable process I should be following to calibrate and fine-tune ever mat I generate?

 

The short version is nothing in the real world is totally black, or totally white, which example swatch colors for certain materials.

 

That's easy enough to follow- thanks!