In case you don’t want to lose time tweaking them, just:
- click twice on the colour picker of the mask exposure compensation (once to reset the value, then once to automatically center the histogram on the -4 EV channel)
- click twice on the colour picker of the mask contrast compensation (once to reset the value, then once to automatically spread the first and last deciles of the histogram between -7EV and - 1 EV)
- display the exposure mask to ensure you get a proper mask (not all black, but not all white). The mask display is there to help you understand what’s going on inside.
Also, as of darktable 3.0rc1, the histogram clipping will be explicitly be shown in graph (for highlights and lowlights) with danger signs:
Finally, notice that the histogram represents the exposure mask used internally. It doesn’t mean that the actual output RGB values will be clipped. If you clipped the lowlights, for example, it just means that the pixels darker than -8EV will get the same exposure correction as the -8EV ones, so you just lost precise control over them, but you will still preserve the gradients in there.