Since the data vs. video levels setting in the video monitoring section is strictly for the video output signal (and doesn't actually change the values being calculated by Resolve) the trick is to always have it set to whichever is appropriate for the display you are using.
So if you are grading on a rec. 709 projector that is set up for full range values, you will want Resolve set to full range. If you then move to a broadcast display that expects video/legal levels, switch to to video/legal, and your black and white levels should feel the same, excepting the inherent difference in display technologies and the sensation of being in a room with lighting. Then you can do a pass to adjust for those differences (in track mode if you feel it works, or on a more scene-to-scene or shot-to-shot basis).
But if you are grading on a P3 gamut, then your DCP workflow might also include a change between P3 and rec.709, which you might approach a little differently.