(For some reason, I could not reply to my own post... So I created this "new post")
This is a long awaited follow-up to my original question.
In order to "progress", conceptually, with RawDigger, I felt I had to go back to basic photography concepts.
So I first purchased a ColorChecker GrayBalance 4"x7" target from X-Rite, having "18% reflectance" which I tested to be approximately the case.
(I can provide spectral measurements if interested)
Next, I wanted to test the relationship between RawDigger "EV0" reference and "real-life photography", in particular "real-life middle gray" or "18% reflectance".
So I took my Nikon D810 out and snapped a picture of my newly purchased ColorChecker GrayBalance target (I still have a Kodak Gray 8"x10" target somewhere?...). I realize I could instead have taken a picture of the 24 patches ColorChecker target and look only at gray patch N5 which also provides 18% reflectance. But I am a "purist" :-)
I thought about setting my camera for Manual Exposure mode and ISO 400.
I adjusted shutter speed and aperture until the Meter in the viewfinder showed "0" (in the middle of the - to + scale).
I took a picture that I then opened in RawDigger, shown here :
As you can see, I did not quite get the darker border in the picture frame.
BTW, it is interesting to note that, on my monitor, I have the impression that the target is not "uniform"? Which I know is not true, having made numerous measurements across the surface...
But "moving" the main RawDigger window laterally on my monitor revealed that it is my monitor that is NOT uniform...
Next, I opened the Histogram window -- what a pleasure :
Ahhhhhh!!!!!!!!!!!!! This is the moment I waited for so long!
What we see in this screen capture is clearly the Red channel centered on "EV0" on the graph!!!!
The two Greens and the Blue channels are not centered on EV0, and that must be indicative of the sensors non-uniform sensitivities? I'm no sure.
One important take-away from the Histogram is the +5 EV levels and -5 EV levels extra, that my camera, supposedly, can capture (?), according to the 0 to 16000 (14bit) horizontal scale? I don't expect exactly "10 EV levels" exposure range from my camera but this is a starting point.
(I confess I'm going to "dig" into the actual "Brightness range" this camera can actually capture)
Last but not least is the EXIF window, confirming the Manual exposure mode setting :
Question : is RawDigger hard coded to "identify" EV0 from the Red channel? It would seem that way...