"Any horizontal surface will generate .5" ?
 I take it you mean... "Any surface with horizontal normals will generate .5"

I'm trying to understand not so much the maths, which I think are a bit beyond me, but the concept. The PM:AO YBias part of the shader makes an assumption that the dominant source of light is directly above, so upward facing surfaces will be most affected by AO. This concept seems to be moving the modelling of directional lighting away from the lights themselves, and putting it into the materials.

Which makes me uncomfortable -- I think that's just because I don't understand the need for it. Is it because of the limitations of AO as implemented in Poser?

Call me simple if you like, but my name is Izi!

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)