Heightfields in L3DT don't have a greyscale range per se, since they contain floating point data rather than integers. The greyscale range is only required when the heightmap is displayed or exported as an image. In such cases, L3DT by default scales the altitude range of the heightmap to span the full range of the greyscale format, with the lowest point being black (value = 0) and the highest point being white (value = 255 for 8-bit, 65535 for 16-bit, etc.)
Using the manual scale option you can get whatever scaling you like. However, it is quite a bit more difficult than is necessary. In the next build I will add a nicer user interface whereby you can set the minimum and maximum greyscale levels. I'll post back here when it's ready for testing.