Have been trying to set up a displacement shader in renderman, but somehow I can't seem to get a decent displacement map out of ZBrush.

As you see in the image below on the top left is the displacement map i got out of Zbrush and did a testrender with the topright image as a result, now I know ( or think I do), that Maya calculates from neutral grey ( which is 128 in r,g and b)so I scaled down to 16 bit tiff and did some grading on the image which resulted in the below two pictures, as you can see that testrender looks a lot better than his predesessor , but stil not nearly good enough.

I use Zbrush4 which has the multimap exporter instead of the old way ( with the code) and I can't seem to get a decent displacement out of it, I really don't like ( bliep) -ing around in PS or NUKE or whatever to get the grading correct I just want to generate it straight out of ZBrush so if someone here could help, then that would be great