::::This is the story behind Ssat = 78/Hsat, according to ISO 12232: 

 ::::The exposure index value (I subEI) is 10/Ha where Ha is the arithmetic mean focal plane exposure. The constant 10 is chosen to be consistent with 2721 and 5763. 

 ::::Hsat is the focal plane exposure that would result from a 141% reflectance object (ie half a stop over a 100% reflectance object). Therefore the focal plane exposure resulting from an 18% grey object is Hsat * 18/141. Put that into the EI equation and you get IsubEI = 10*141/18/Hsat = 78/Hsat. The speed is the EI for an 18% grey card, hence Ssat = 78/Hsat