Uniform skin color is difficult where ambient light is reflecting off brightly colored surfaces.
Skin color also varies in different areas of the face.
Eye accommodation after working at a monitor and differences in lighting between the monitor calibraton and reflected light off prints make for a difficult situation.
Skin colors vary widely, but a rule of thumb for CYMK values of Caucasian skin goes something like this: Cyan about 1/2 the value of Magenta and Yellow slightly higher than Magenta (~10%)
Cheeks, nose, chin are often redder than other areas, so that should be taken into account.
I made four selections to compare, then did an overall adjustment with levels, followed by a color balance adjustment. My 4 selections fall pretty close to this criteria but may need a reduction in the yellow.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=39170352
original image by cberry: