Don't let yourself be too bothered by exact values for one particular solution when starting out. Those values are meaningless in any other shader than the one you are using. It is better to spend that time understanding the basics physics surface and light interaction. The best points I can give you are:
Understand the range of Index of Refraction that maps the specular slider to. (1.0 - 1.8) is the original Disney baseline that everyone is starting with. IOR tables all over the internet. You can use those as reference data. When in doubt, just go with a fixed 0.5. Roughness matters more anyway.
As far as roughness or glossiness goes, in general, surfaces are generally glossier than most would probably perceive them to be. The big thing here is the representation of the highlight. Can you clearly or semi clearly distinguish the reflected image / object? Very few surfaces end up in the upper range of roughness as well.
For metals, these can be tricky, but the good shaders map the color as a 0-1 value, per channel, as a colored index of refraction entry value (meaning the color is a % reflective when the view angle and surface normal are in alignment.) When you know, that and spend the time finding good metallic color reference, the PBR values for your shader can be easily had if you know how your shader is interpreting those values.
In summary, PBR values are not concrete and are not a fixed target across multiple shaders and renderers. Many shaders can made to be data referring, the specular slider is a good example. But that is only if the range under the hood is not changed. Understand your shader and physical light interaction as much as you can, and arm yourself with data and reference. In the end, just make it look good though.
Oh, and depending on the function that metalness uses to determine metallic reflections, it can be much more "physically correct" and also more intuitive than a specular color and intensity slider
Replies
Check this:
http://seblagarde.wordpress.com/2011/08/17/feeding-a-physical-based-lighting-mode/
http://www.fxguide.com/featured/game-environments-parta-remember-me-rendering/
http://www.marmoset.co/toolbag/learn/pbr-theory
http://www.marmoset.co/toolbag/learn/pbr-practice
"PBR is more of a concept than a strict set of rules, and as such, the exact implementations of PBR systems tend to vary. "
Understand the range of Index of Refraction that maps the specular slider to. (1.0 - 1.8) is the original Disney baseline that everyone is starting with. IOR tables all over the internet. You can use those as reference data. When in doubt, just go with a fixed 0.5. Roughness matters more anyway.
As far as roughness or glossiness goes, in general, surfaces are generally glossier than most would probably perceive them to be. The big thing here is the representation of the highlight. Can you clearly or semi clearly distinguish the reflected image / object? Very few surfaces end up in the upper range of roughness as well.
For metals, these can be tricky, but the good shaders map the color as a 0-1 value, per channel, as a colored index of refraction entry value (meaning the color is a % reflective when the view angle and surface normal are in alignment.) When you know, that and spend the time finding good metallic color reference, the PBR values for your shader can be easily had if you know how your shader is interpreting those values.
In summary, PBR values are not concrete and are not a fixed target across multiple shaders and renderers. Many shaders can made to be data referring, the specular slider is a good example. But that is only if the range under the hood is not changed. Understand your shader and physical light interaction as much as you can, and arm yourself with data and reference. In the end, just make it look good though.