You could figure out the exact number for 127/255 or 128/255, but you can also just use Poser's color picker to sample the displacement map in a neutral area.

Use a Color_Math:Subtract node, and for Value_2, click on it, then click on a neutral gray area of your displacment map in the preview. This will copy the exact neutral value into the subtract node.

So it is automatic this way.

You can make any value you see be the neutral value just by pointing at it. Easy peasy.

Because the two squares in the neutral areas are perfectly aligned, we see a goofy pattern where Poser cannot tell which one is closer to the camera. That is good - we did it right.

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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)