Reader Brian K wrote to ask why I was shooting ISO 31 on the Nikon D850 monochrome, since ISO 31 is a fake ISO and no different than ISO 64 (raw file is the same).
The answer is shown below: by shooting at ISO 64, the histogram get jammed over to the right, which at a glance is much harder for me to evaluate for fine tuning that last 1/3 to 1/2 stop of exposure out in the field. The camera will also blink highlights a bit prematurely.
So bottom line is that since the raw file is the same, I want the easiest to grok histogram.
In color, this may be less of a good idea, since Adobe Camera Raw might behave slightly differently (not sure) even though the raw data is identical for the same exposure. But in monochrome, it is exactly what I want.