@reyalp After pushing out the last post, I've tried to educate myself on gamma vs linear, eg
https://www.imatest.com/2021/07/tonal-response-gamma/I've convinced myself than the viewport based CHDK (and Canon?) histogram are gamma encoded at 2.2.
Thus the top 1/3 stops look like this: 255-229, 228-206, 205-185.
This follows from: 256*2^(-(n-1)*(1/2.2))
