Skin translucency

[figure1a]

The KS manual states that this for the translucency setting for translucent materials "This controls how deep light penetrates into and through the surface depending on the units set under Edit, Set Scene Units. The higher your translucency value, the more of the subsurface color you will see coming through the surface. Higher translucency values will also create a softer looking material."

So, for skin, I would think that it would be set to like .005" or something, right? But I continually see samples of people setting 8 feet or 30 feet or whatever. I just set a skin material and if defaulted to 3.67 feet. What is it about this that I'm not understanding? I have no idea where to set it and usually waste a half hour so randomly picking numbers.