Yes, medium gray is considered zone V.
Caucasian skin normally falls in VI-ish.
(Roman numerals are normally used to denote zones because zones don't always correlate equally to stops.)
The classic correction/offset when metering Caucasian skin is to increase exposure by 1-stop.
My caveat is that it depends on the lighting, the skin that you meter from should not be in shadow nor in highlight.
If you have, or can borrow an incident meter, you can find the classic camera setting that will give you "normal" zone placement, then you can meter various points on the face to find the various zones and variations on the face, on a grey card, or on some other reference point.
A bright side cheek may read in zone VIII, dark side zone III.
That variation is simply the nature of the beast when reflective metering is used. (It is also why, when given a choice, I almost always incident meter.)