why not make a scene with a simple emitter, or several - at different known luminosities, render it out, read out the values and use it as reference to obtain scale values?
I remember vaguely in the new sky texture thread, there was some discussion going on about scaled values - and what blenders light multiplier of 1.0 means in real world values… you might find something there too: New Sky Texture
EDIT: found it: New Sky Texture - #310 by nacioss