I'm not sure, about that...
I think, "medium" (non-changing) values are r .5 g .5 b 1 (or r127 g127 b255)...
but if you convert a bumpmap in a normalmap, the same amount of detail would be there, compared to the direct normalmap convertion...
And Indigo doesn't feature normalmaps, so there's no need for this (until Indigo supports them)
Even more interesting would be the HDR versions of the Normalmaps:
Vector Displacement Maps
(there, black means no change, I think...)