The best thing about this Thread: I have an answer of what I have to use in Substance Designer to get a F0 Value.
My Question is now: Is there a place where I can look up what output in Substance refers to what kind of desired map?
What I mean is that I really have bo idea what the differences are between the specular, the specularLevel and the specularColor outputs. There also is a reflective output and it confuses me a little 
Hi Fabian,
The good news is that volume 2 will fully explain all of the maps and processes. It should be available next week : )
Specular has a different meaning for metal/rough or specular/gloss workflows. In metal/rough, the specular for dielectrics is hard-coded to be 0.04 (linear value) or 4% specular. They shader handles this. For metals, you then place the metal reflectance value in the base color map and the metal map indicates to the shader which areas are metal and which are not.
With metal/rough, you can override the hardcoded value by the shader using the specularLevel output. This is just a output that tells the shader to use a different value for dielectric F0.
In PBR, you don't use the reflectivity output. For specular/gloss workflow, both the dielectric F0 and metal reflectance values are placed in the specular map.
So with PBR you have either metal reflectance in the base color indicated by a metal map for metal/rough workflow or a specular map for dielectric F0 and metal reflectance values for the specular/gloss workflow. The specularLevel is an extra level on control for dielectric F0 for metal/rough workflow. With specular/gloss, you already have full control over dielectric F0 in the specular map.
Cheers,
wes