I don't completely agree, because I get very good approximations with all types of photographic materials by using non-linear curve-fitting algorithms of 3rd to 5th order. The following equation was used to fit an ISO grade 4 contrast for MGIV-FB:
R^2 = 9.999E-1
Feel free to plot this for x-values from 1.0 to 2.0 to see for yourself. As you can probably tell, I used the following equation format:
Unfortunately, this is not helping the original question, but I'm very interested to continue this conversation via private eMails, if you like.