Skin tones vary wildly, so going by the numbers can only get you into the ballpark at best. At some point, you are going to have to learn to eyeball things (where "eyeball" is understood to be a subjective process carried out on a calibrated monitor so that you have at least a reasonable expectation that what you see on the screen is living in the same universe as what you're doing to the file).

Lightroom (and Lr isn't alone in this) is somewhat crippled in this regard because it only has RGB numbers to look at. But that should be enough to get you into the right range, at least in terms of exposure. (Actually getting the skin colour right when there is no colour reference in the photo is a lot easier when you can deal with standard CMYK -- which depends on the selected output device -- or LAB -- device-independent -- values.) In general, though, you are looking for a colour in a non-specular (that is, not "shiny") broad highlight area that has a red value somewhere in the 180-220 range (depending on complexion), with green and blue both being significantly lower (140-ish to about 180, again depending on complexion). Foreheads and chins are usually good places to look for such highlights; the cheek can have some makeup weirdness going on that makes the normal RGB values untrustworthy. (Keep in mind that you can always go to B&W temporarily to take colour out of the equation.)

But seriously, get your monitor to a place where you can trust it, then you can trust your eyes. A beach bum in late summer should not have the same skin values as a light-averse fashion model in midwinter even when they're shot under identical lighting. As an experienced shooter, you'd meter them differently (with a reflected light meter, such as the one in your camera). If you try to go by the numbers, you lose a lot of nuance.