OK, so we are making whiter skin whiter and darker skin darker? I understand that middle greys don't look too good for white skin, so we want to over expose a little, but I am a little unsure that you would want to under expose with darker complexions.
Or am I mis-understanding?
I don't think you're underexposing if you adjust the exposure to a stop less for people with very dark skin. It depends by the person skin tone, but I find middle grey to be a better translation in black and white of the skin tone of people with lighter dark skin (if that makes sense).
Personally I use an incident meter and place it under the person chin towards the camera.