Load the disp map into some tool where you can eye-dropper the neutral gray. It may not be exactly 50%. Suppose it is 51%, then you need to subtract .51 to bias the neutral level correctly.

---

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)