I don't understand what the radius has to do with it, but if you just want a length of 0 = 255 and length of 10 = 0, all you need is (1 - length / 10) * 255

That's exactly what I needed, thank you very much.

Just one more question: Could you point me to a source of information that discusses the formula you posted? I don't mean to bother, but I always like to have a conceptual understanding of the things I work with.

Once again, thank you very much.