My current method is:
1. I choose a reflective curve of a material I want to create.
2. I represent this curve in fallof map that is put in the reflective (fresnel is off).
3. I'm trying to eyeball correct setting for GGX (GTR tail fallof) but im not sure what i'm doing there, what to type in there.
I do not blend materials with different gloss. already since i heard that GGX can do this for me. But i'm still trying to re-create the propper fresnel. Is that method ok?
Unfortunately, this won't re-create the proper Fresnel, unless glossiness = 1.0 or so.
It's far from easy to explain why it's so, but I'll try.
The problem is that micro-facet models (including GGX one) use Fresnel a bit differently.
Fresnel equation/curve assumes a perfectly smooth material.
Micro-facet models assume there a lot of micro-facets, oriented differently, i.e. their normals differ from the surface normal, according to some distribution (e.g. GGX distribution).
And a micro-facet model integrates responses from the distribution of the micro-facets in some nontrivial way.
The main problem is the Fresnel effect is calculated using so-called half-vector (a vector in-between a light and a camera rays), not a surface normal vector.
So, it's sad to say, but one cannot re-create the proper Fresnel in many renderers for rough materials.
Basically, there is no way to access light rays directly in a typical path-tracing renderer node system.
Some renderers implement micro-facet models in a "separable" way, i.e. they completely ignore the Fresnel term during light integration and multiply by a "smooth" Fresnel term later (using normal vector). Which works okay for a glossy case, but results in a noticeable unnatural rim in a rough case (glossiness = 0 or so).
This cannot be easily solved with falloff maps and so, since a proper implementation will use a different Fresnel curve for different light rays, roughly speaking.
So, a typical solution is to adjust GGX parameters (possibly with some curve/map) or even mix two GGX materials in some way, so that it looks plausible.
And this is unlikely to change, since it's non-trivial to implement a flexible solution. Ideally, there should be a dielectric GGX implementation and a metal GGX implementation, and probably a separate tinted-metal GGX implementation :). With enough parameters to calculate proper Fresnel curves during light integration.
Hope my explanation will be helpful, but generally speaking it's a quite complicated matter (I've spent lot of time to figure out what is the problem with the crazy rim effect for rough materials)