I was playing with the newly release C4dtoA plugin for Cinema 4D and was a little dissapointed that the shader library is very small. I wanted to be able to map my own fresnel curves with information from refractiveinfo.index, but to do so you need to know what normals are facing the camera and what normals are perpendicular to the camera. That's exactly what cs_facing_ratio does. It calculates the angle between the ray shot from the camera and the normal at a given point, and maps the values from black to white (black being parallel, white being perpendicular). These values can then be remaped through the ramp_float node (ships with Arnold) with the values of a fresnel curve.
This node is also useful for mapping things like the roughness of your freflections, since in reality reflections apear more rough at a 90 degree angle and almost mirror like at a 0 degree angle.
It's totally cool to use this in any project or job you'd like, please be sure to credit me if you do so. Also let me know if you make anything cool! I'd love to see it.
DOWNLOAD V 1.1 (6/26/2015):
Drag and drop the shader file into $Arnold_Plugin_path/shaders/
(Ex. /Applications/Maxon/Cinema 4D R16/plugins/C4dtoA/shaders/)
Shoot me a message if you have any questions or bug reports!