Virtually everyone on Wall Street knows that Bill Ackman's big short bet on Herbalife has cost him so far. Now we know exactly how much it has declined: by half. Ackman's $11.96 billion hedge fund firm, Pershing Square Capital Management, first took a short position in the nutritional supplement company on May 1, 2012, according to a presentation at its annual investor dinner Feb. 13. Since then, the firm's cumulative loss on the stock has totaled 49 percent, representing Ackman's worst investment ever. The dollar amount of the loss is unclear.