Of course, most people expose skin in music videos and in fashion photography very differently than in narrative naturalistic films. What I am concerned with at the moment is skin exposures for narrative films. I am concerned with what exposures will render the skin most natural.
My spot meter is calibrated so that the f-stop reading will give me 50% middle gray in the camera. I have observed films like White Material by Claire Denis, Ballast, Black Swan, I Am Love, Ondine, among others and have studied their exposures with the waveform display in Final Cut Pro. I studied mostly White Material because it was a good mix of dark and light skinned people in the same frame.
From what I have observed, it seems that most fair skinned people are exposed at key/middle grey or 50-60% brightness in indirect light or soft ambient light. When they are in direct sunlight they are commonly exposed at 70% brightness or one stop over key. Sometimes I see 40% or 1/2 stop under key in darker shade.
Of course, the shadow regions on their face can go as low as 0% depending on creative decision. I am not taking shadows into consideration because they can vary so much. However, the main light (key light) is often consistent to the percentages that I have noted above.
Most dark skinned people in narrative naturalistic films are exposed at around 30-40% brightness in indirect light and around 60% in direct light. (However, in music videos dark skin is often exposed at 50% in indirect light and three or four stops overexposed for the rim light)
Here is a small table (this is in no way an end-all-be-all fully comprehensive, but just a table of averages based on observations):
Fair skin:
Direct light - 70%
Indirect light - 50%
Dark skin:
Direct light - 60%
Indirect light - 30%
Any comments or corrections would be very welcome.