Equivalent Mix fac to Fresnel? 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?
If you're looking to calculate the Fresnel factor by yourself, a commonly used approximation is this: https://en.wikipedia.org/wiki/Schlick's_approximation

This should give you a percentage of how much "gloss" there should be.