The purpose of the bump shader is to map the lowest altitude to 0 intensity and the highest altitude to 255 intensity. If you want some minimum value to be black and some maximum value to be white, use Texture>>Shader Setup as above, but set the Land altitude min and max to the values that you want (for example, -1000 and +1000), set the Sea level to -1000000 and abs min to -2000000 (in short, set them to values far lower than will ever happen in your surface). The click the Color List button on the land side, hold down the Shift key, and click Remove to remove all entries. Then click Add and pick black, then Add again and pick white. Click OK and OK again to take effect.
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If you want to have pure heights without any apparent lighting, use the General page on Lighting Settings and select Display Type as Height Code.

To save this lighting setup, use File>>Save with Lighting Definition as the file type.