The Z maps should be relative to what's in the image, dividing up the closest to the farthest into 256 steps regardless of the actual distance between them.

One thing that can screw you up is to make sure that there's no object far in the background that is throwing it off. I've done more than one render where everything turns out almost white because I didn't notice there was a tiny gap in the background allowing me see a sliver of a skydone or something similar which screwed it up.