Hi! First of all what kind of dispalcement map do you have? It's made with zbrush displacement native maps, or is done with the alpha displacement exporter? I made displ maps using the ADE. Why? Well zbrush displacement maps have no scale imbeded so to speak.that means that you back in maya have to find for yourself an alpha value that is apropiate.With ADE you know that the alpha value should be 2.2 in maya, if you checked the adaptive option in ADE (i don't have time to explain all of that).To balance the 50% gray, you have to set your alpha offset to -50% of your alpha gain value, through an expression : for ex. file1.alphaOffset = -(file1.alphaGain/2);.So just right click on your alpha offset field and choose expressions and write like above.Wish you luck.If you have 10$ you can get from gnomonology website a wonderfull tut by scott spencer.(if you have luck with an torrent account.....)