xnormal has hice tools,like heightmap to normal map etc,and also a precious tool "ray distance calculator".
This tool gives the distances the rays travel from the lowpoly to the highpoly reference model to bake details
(it is the differences in units between the two models).
http://www.xnormal.net/1.aspx
And also important,if this tool can give you the correct min and max height between the low and high poly models,you can use these values to adjust the height of the normal map if you are converting a heightmap to a normal map with the nvidia tools or whatever.
I mention this because
1.The heights must be correct because the sharp edges of the low poly need that specific heights so they smooth according the high poly,and
2.the heightmap can easily get more details than the actual high poly 3d model (it would need over 15 million faces to simulate an average detailed heightmap).
The best way is to 3d paint (to avoid the UVW distortion) the extra bump details on the 3d model using a layer over the original heightmap.
Then you use the layer with the details as a mask to raise very slightly the brightness of the heightmap.
Then you convert to normal maps.
Heightmap to normal map conversion is mostly for level geometry and terrain.
Textures that are to be seen from afar on mountains might need normal map height adjustments of 20 units and more i d say.
For the moment i use a photoshop nvidia normal map filter plugin,which makes good maps with the default settings,and you just need to adjust the height.