Custom Fresnel with separate wavelength input How can I create a Fresnel effect taking in consideration the custom wavelength curve generated on refractiveindex for a more accurate fresnel effect in cycles.

. The simple IOR input isn't reflecting accurately enough what happens in reality.
I can think of two approaches, either adjust blender's code and build your own version. You may even be able to get your changes accepted into the official blender sources.

Or write an osl script that calculates the way you want. As a starting point the official fresnel node is implemented as -

shader node_fresnel(
    float IOR = 1.45,
    normal Normal = N,
    output float Fac = 0.0)
{
    float f = max(IOR, 1e-5);
    float eta = backfacing() ? 1.0 / f : f;
    float cosi = dot(I, Normal);
    Fac = fresnel_dielectric_cos(cosi, eta);
}


And you can find fresnel_dielectric_cos() here