Meaning of greyscale values in "Bump Maps"
The greyscale values in "Bump Maps" don't indicate a certain height. Only the relation in brightness of the shades of grey of two adjacent areas serves for the "Normal" calcution. A "Bump Map" with just one shade of grey or with only white or only black looks completely flat on a plane. Normally, a transition to a brighter shade of grey stands for a "Bump Up" and a darker tone for a "Bump Down".