Spec is basically the index of refraction of a material. The old values are not needed since the new system is energy conserving which means: roughness controlls the shape of the spec reflection and the lights intensity defines the spec power. Most realworld materials have a IOR close to 0.5 that is also the default value for spec if nothing is specified 
I would suggest learning the theory behind pbr to fully understand how it works.
As soon as you got that, things actually get way more easy and fun than it was before