Well normalmaps are rather complicated. Each point in the "image" is actually an angle so my guess is that the information is stored using all 4 parts (RGB and A) to create a floating point value in degrees or radians.
the maths involved in converting a simple height map to a bumpmap makes my head hurt just thinking about it. You can`t just take each point and convert it. You have to analyse all the surrounding points so you know if this point is on an upslope or downslope and how steep the slope is before you can start calculating the angle of the surface normal. Lots of nasty 3D vector calculations
I think the FSX sdk recommends using the nVidia plugin/tools for creating normalmaps. Nothing I could come up with is likely to be as good as what the professionals have been doing for years
