Question Old specular (SurfaceOutput) vs new Metallic PBR (SurfaceOutputStandard) specularity maths

I have a project stuck in the past and was wondering how the maths for the new specularity changed in the metallic standard shader - it has a curious specular highlight which shrinks near the end of the smoothness curve (I'm not even sure where this idea of light behaviour comes from!).

The old setup had gloss and specular values but mainly amounts to a more and more focused & bright highlight forming.

Focusing just on the smoothness (or gloss) of the standard metallic PBR shaders, the specular highlight is slow to appear and then goes quickly bright before shrinking in size as reflection comes in!
Can I find a reference on the maths of that somewhere?

 

wrong section - should be shaders if Mods around!