luminance/lightness channel preview for setting exposure, compare to theme?

The manual recommends getting mid-tones right for scene-referred workflows first. I have also read that darktable-elegant-grey is near middle-gray so it is useful as a comparison.

I have a couple of questions though — apologies if they reflect some misunderstanding.

  1. Are we talking about the J channel?

  2. Is it possible to preview the lightness of the image (the J channel) directly while adjusting exposure? I am aware that I can open the parametric mask controls and press c over the relevant channel, is it would be useful to just view that while adjusting exposure.

  3. Which part of the theme should I be comparing middle grey to, the area surrounding the image in the darkroom?