Hey man, I've been afk the last couple of days.
I got 91 flat planes that rotate from 0 to 90 degrees.
"Environment Map" is pure White and the material has Black diffuse.
I render front view in orthographic, select the "CESSENTIAL_Reflect" element and sample the 0 degrees plane.
You can do this quick calculation and compare it with the sampled data.
Reflectivity = ((1-IOR)/(1+IOR))^2
Example:
Reflectivity of 1.5 IOR = ((1-1.5)/(1+1.5))^2 = 4% | Corona 4.03%
Reflectivity of 50 IOR = ((1-50)/(1+50))^2 = 92.31% | Corona 92.90%
Edit:
Siger ThinFilm just for fun
and here is the difference between "1.0 Reflection" and "0.5 Reflection"