I found a reference to an Ansi standard # PH3.49-1971 that supposedly sets the standard to 12% +/- 2%. I tried checking the Ansi website but it seems you need to be a member to find anything.

I found this:

http://members.cox.net/dspielman1/Gr...H3_49_1971.PDF

From David R. Spielman Brooks Institute of Photography