There is no such thing as a formula for skin tones. The range for every type is limitless. What you want to do is to orient your thinking along the lines of relative amounts of each color. Even if you work in RGB it's not a bad idea to think in terms of CMYK values for skin tones. The colors in the RGB color space that are out of the CMYK gamut are not ones that you ordinarily associate with normal skin tones: brilliant blues, greens, oranges and reds. Even when I'm in RGB I'll have my Info readout set to CMYK; it's easier to keep things in perspective.
Caucausian tones run the gamut from pale pink to darkly tanned. Rule of thumb is roughly equal magenta and yellow, with yellow favored, and about a fifth cyan. Pale tones can have magenta and yellow values as low as 25-30%, whereas a heavy tan can jack the numbers up into the 60s and 70s, with yellow racing ahead of magenta, for the rich golden hues that magazines like Maxim and FHM like. Cyan becomes more important as the tones darken, pulling back the tendency for shadows to turn hot. A good median tone might be 10C 50M 54Y. Take your readings on cheeks, but not where the light hits directly. Be prepared to use curves, not levels or color balance. Skin that looks too yellow, or too red often needs more of the offending color pulled out of the highlights and quartertones than in the darker areas.
Darker toned flesh, hispanic or Asian, is similar to tanned caucasian tones. Cyan is higher in relation to magenta, and yellow higher still.
Black skin tones in general have far more variation than other types. As you move toward darker tones, cyan becomes more important and rises proportionally in the mix. In a 4 color separation, black ink usually comes into play only in the darkest areas of shadows, so if you are in RGB, you will not want your shadow areas to neutralize. Pull back on the yellow. Darker black skin shouldn't favor yellow as much as with lighter skin tones. A reasonable set of values might be 59C 75M 69Y 18K (Corresponding RGB values would be 100R 62G 60B) whereas lighter tones could read 30C 55M 62Y 6K. Note the low value of black in relation to the other tones. Black is for shape, not tone.