How about a different approach.
In this case - I created 3 colour ramps (R, G and B) based on this graph from the link below
http://markkness.net/colorpy/ColorPy.html
I then added them together using the layer weight node key the observed thickness as a function of viewing angle.
I wonder if it might be possible to recreate the graph using a simple math function for each RGB component, rather than using colour ramps. The three RGB components appear to be based on a sine wave with a bit of modification at the start and end.
Perhaps a sine function multiplied by an RGB curve…hmm have to think on that one.