EM = 1/2(UP*IV*Sqrt DUE) / Sqrt 252
UP = $520.45
IV = 17.11%
DUE (days) = 12
Sqrt DUE = 3.46
Sqrt 252 = 15.87
EM = $9.72
This means that 34% of the time, $OEX will move $9.72 in one direction. The other way to interpret this number is that 68% of the time, $OEX will stay within the range of $510.73 and $530.17.
This is the best way to push your luck over the long term if your position is threatened and need to "sweat it out".