The thing that threw me off is that f0 seems to affect overall specular amount similar to the specular node while f90 doesn’t seem to.
As you can see f0 has a dramatic effect on overall specular values while f90 doesn’t. Though that may be intended.
Based on the description I was expecting an effect more similar to what’s shown in this video Specular & Roughness control for Unreal Landscapes - YouTube though I guess I may have just misunderstood.