Normally we use Fresnel (either from the node or layerweight) to mix between Diffuse and Gloss shaders. This works best for normal shaders, but isn't always best for non-realistic shader setups, or multi layer materials. I am working on a shader setup that uses normal shaders mixed with fresnel, but also includes toon shaders with a flat mix fac.
I want to establish a relation between Fresnel IOR and a normal flat mix so that I can derive the flat mix from the IOR. In other words, I want to take the IOR input from my node group, do some math to it, and end up with a flat mix for my toon shaders that will be about equivalent in shininess to the IOR.
I understand that "shininess" isn't a particularly precise descriptor. Is there any particular math, or just general rules of thumb I can use, or do I just need to eyeball this?