This… A thousand times this.
Almost always the ‘embedded JPEG’. i.e., what your camera would call ‘the final picture’. Like others have said, maybe with alternative firmwares, but I don’t know of a camera that has a RAW histogram out of the box.
My easy - and harsh - answer is: ‘you cannot relate’. And maybe you shouldn’t even try.
Things like that ‘D+’ setting you talk about. Some cameras have modes like this that actually alter the exposure and compensate in the ‘developing the picture’ step (so they actually shoot darker than you think, but compensate the final JPEG file. This way you preserve highlight data, also in your RAW file). Others just adjust the curve they use to make the JPEG (so the tone curve they apply will leave more detail in the highlights, but the shot itself is not technically different, so the RAW file should be the same, which means you still need to be careful not to clip your highlights).
See it that your camera has a ‘built in RAW converter’, and the histogram you are seeing is for what the camera is doing ‘in the edit’. Darktable is another RAW editor, so will do different things to your captured light-data, so will display a different histogram.
I always walk around at -1EV or even more by default on sunny days. But I know that I’m more allergic to clipped highlights than to noise, and my cameras doesn’t add a lot of noise in the shadows.
Your D70 is one of the older Canon’s that’s notorious for noise added to the shadows, even at low ISO’s. So what you might want to do depends on how allergic you are to noise or to clipped highlights. That’s something only you can decide.
But that D+ mode is a typical ‘make shot too dark, compensate for it in the edit’ from what it seems like. So it’s only natural that you have to do the compensating yourself when you open the RAW in Darktable (because the camera only did the compensating in JPEG, not in RAW so to speak).
I would happily use something like that, but as I said, I don’t mind a bit of noise, or I’m not afraid of it. I can’t decide if that’s true for you, or how big the noise problem is on your camera, etc…
Try it out, is all I can say. Experiment a bit.