The same was true for accurate reflections. I experimented with edge_blend, bias, and gain, and other stuff for MONTHS. Then I quit horsing around and looked up the equations for Snell's law and the Fresnel effect. After 20 minutes of coding up the math, I instantly had a totally perfect model for reflection. No tweaking.

On the other hand, when I set about to make a shader for leather, there were no equations at hand that I could look up. So I did a lot of experimenting in the material room. However, a combined approach was called for here. I had to "discover" the rules of wrinkles in leather, and expressing them took a lot of nodes. I couldn't have arrived at the solution through staring at the preview or renders alone. It was too much clicking. So I'd try different math formulas in matmatic, and study how they looked in the poser surface preview and with test renders. All three techniques are valuable - guessing and rendering, pure math, and a combination.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)