For doubling 11 vs T in an ENHC game, +4 would be the EV-maximizing index and +12 would be the risk-averse index. I don't know if that's what Wong was thinking when publishing +12 in the 1981 edition. Don and I discuss this play in our book The Hi-Lo Card-Counting system: A Complete Guide to Index Play.
Another difference between the two editions is that Wong used truncation for true count conversion in the 199 edition, so all of the negative indices are higher by 1 in the 1994 edition.
by: Gronbog
Thanks, that helps to explain this.