How do we evaluate the values a skin (or other material) should have?

We know this from real life measurements, and optics laws in physics. As a result, these things come with photoreal renderers like LuxRender. And the word is spread by people like BagginsBill, SnarlyGribble (and me, and more). The more you're into photoreal the more it comes to those details. But for most artistic purposes, the good news is that the eye is not too critical. You can survive a long time on decent rules of thumb. Now we only have to make this accessible :)

How do you calculate for Blinn etc?

I hardly do, because most of these aspects have to do with the way light from an angle is handled by curved surfaces, like the difference between smooth plastic and rough clay. But in some cases things get awkward, like finding the proper balance between specular and reflection. Again, the closer one approaches photoreal...

Lighting & materials

yeah, real lights have a size, we do live in an atmosphere, undera sky, surrounded by light diffusing walls and objects do scatter etc. Without SSS and IDL, things keep looking like metal balls on the moon shot with a flash. Photoreal lighting and surfacing is more a career than an action point.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though