The Omega II Count
Omega II, from Blackjack for Blood, adjusts the level-2 formula. Cards 4–6 count double, the 9 counts −1 (the only major system that prices it), and the ace sits out of the main count with a side count for betting. Its measured performance is close to Hi-Opt II's. Choosing between them mostly comes down to which book you learned from.
Same prerequisites as the other level-2 courses: a fluent level-1 count first.
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