Hi there,

When in Zbrush try to use Displacement exporter instead of zmapper.

Also Normal Maps are mostly for Game environments, for best results you should use displacements.

So is you still want to use Zmapper,

Then use the normal map configuration, maya7surfacesampler
Then from the texture channel, create a grey scale.
Export the greyscale version,

Open up PS, and because the grey`s in zbrush are set to 50%
You need to change the value in PS.
And that can be done with the CTRL L function.

here is more info https://206.145.80.237/zbrush/zbcfiles/ZMapper_Doc.zip

Also render with MR, and setup a Approximation node.

Window>Rendering Editor>Mental Ray>Approximation editor

Then create a polygon subdivition.

and apply the settings found here> https://www.zbrushcentral.com/zbc/showthread.php?t=43264

Hope it helps.