I’ve re-processed the gradient images, this time white balancing on the backlight, here are the curves:
If we “normalize” the values by multiplying by some factor, to make the curves “start” at the same point, and compare them to the previous curves with WB on the film base, we see that there’s also a slight change in the slopes:
This is most probably caused by the “mixing” of channel values during input profile conversion, which sums a fraction of one channel into the others.
Here is the “inversion” function fitting the new, backlight-balanced data:
… and here is the result, using the new parameters from the above plot. The picture was also white-balanced on the backlight:
Definitely not good, still needs some manual adjustments.
I’ve noticed that there is also a simpler function, that might fit the toe part a bit better:
exp(SHOULDER * (-x - LINEAR)) + (-x * SLOPE) + TOE * log(-x)
I’ll try this one, too 