Hi, thanks for describing your method for negative inversion. This showed me, by the way, the answer to my question on how to set the shadow and highlight points in RT for the three channels .
As far as I understand the white balance algorithms, the most common use is the grey world approach, which aims at the same “centre of gravity” for the three histograms in R, G and B. Setting the white and black point accordingly is what I usually do, just like you described.
The problem is, that you need a patch on the negative, which is not exposed, i.e. exhibiting the clean orange mask. Otherwise it is only an approximation what you get.
PS: The method for inversion used by ColourPerfect is described in a paper by David Dunthorn.