Ive started a google doc with specular range RGB values for various material types. The ranges here are gather from various resources that are available publically and a few that I have gathered using cross polarization photography. Im currently building a scanning rig to expand the library.
The values there right now are in no way 100% accurate but they are good approximations, may take some tweaking to look perfect.
I thought it would be good to make it public so if people wanted to add their own values they could. Just leave a comment and I can go ahead and add to the list. And of course if you have different values I would be interested in hearing them!
Here is the link