It seems to me like a back-justification of something not measurable analitically.
A 5 millions US$ bracket is not in any way an approximation.
IF you had narrowed the interval (and IF the picture is at the auction sold within it), then all your data mingling exercises might have some usefulness, otherwise it is just an arbitrary estimation like the one from Christie's.
So the similarity calculated by an arbitrarily chosen software between (photos of) study and painting of improvisation #8 is 64%.
The similarity between (photos of) study and painting of improvisation #20 is a MUCH lower 57%.
It is clear how this HUGE 7% different can easily bring down the value of the estimation by more than 50% from the improvisation #8 sale price of 23 millions for a 98x70=6,860 sqcm painting, i.e. 3,350 US$/sqcm to a mere 10 millions for the improvisation #20, a 71x99=7,029 sqcm painting, i.e. 1,423 US$/sqcm, give or take a couple of millions.