Coming back to this - thanks for the tip BagginsBill, Stonemason, that was giving me trouble.  The map exported from Zbrush has 50% grey as the neutral point, and subtracting .5 did the trick.  There is a faint line at the edge of the map, most noteworthy at the ear, because the adjacent material zone does not have a disp map itself, but I can live with this.  Excuse the very rudimentary craftsmanship on the map itself, it's a quick first effort.

My Freebies