If you activate the tone equalizer in darktable, it displays EV values for the part of the image the cursor is placed on. can this be used as an accurate way to measure the dynamic range of the scene, assuming that the entire image falls in the -8 to 0 scale used in darktable?
No, because the EV displayed is for the guided mask, not for the underlying image.
I think this is just the mask and not the image…you have an 8EV mask that you can adjust to cover parts of your image…changing mask settings give you a completely different value and there is no change to your image…
Okay, thanks guys! Makes sense!
Have never understood why editing software can’t tell me where a part of the scene falls in EV. Raw Photo Processor does and being able to define a reference point (white clipping, middle gray, etc) is a beautiful thing. I mean, everything we do in capturing photos is all based around EV steps but once the image is on a computer there’s no connection to the decisions you made metering the scene. It’s one of the biggest roadblocks for learning digital photography and image processing. Everything in the user interface side of things should be scene referred logarithmic EV instead of linear or display gamma. Would make everything so much more intuitive.
filmic is based around a mid-grey point, with you specifying your desired black and white points in scene-referred logarithmic EV. exposure allows you to adjust exposure correction to move a desired area to mid-grey.
One thing to keep in mind is that EV is a relative scale. That means you can only express differences in EV, not absolute values (at least in the way dt uses EV). So unless and until you define an anchor point, assigning an EV values isn’t possible. In dt, with the scene-referred processing, that anchor point is the middle gray value, set through the exposure module (cf. the post by @kofa ).
Wrt. the tone equaliser: that module uses a mask with a fixed width of 8 EV, mapped on the tonality range of the image. That also means that the range covered by the mask can be more or less than 8 EV in the image (when you start playing with the “mask contrast compensation”).
And don’t get me started on “intuitive” interfaces 
