I use the thumb rule that says caucasian skin, placed at around zone 6, usually gives the "correct" exposure and pleasing tones.
This means, if you take an incident reading, set a camera with a spot meter to this exposure and then meter a middle tone in the caucasian face, the spot reading will usually suggest that this reading is 1 stop over "the middle notch" in you camera.
By taking an incident reading, you are establishing an exposure that will place 18% gray in zone V (middle gray in the middle). When you do this, caucasian skin which is thought to be one stop brighter, will be placed at zone VI and thus your exposure will render skin naturally.
I've found this to work very well in natural light portraits, especially outside when there is overcast weather.
Off course you can be anal about it and state that makeup, light hardness and direction will alter the highlights in the skin for example, but if you have nothing else, make an incident reading and shoot.
(Incident readings should be measured from your subject, in direction of your camera)
My two € cents...