http://www.irs.gov/pub/irs-pdf/p15.pdf
Page 15, paragraph 7.1 a&b.
Ultimately it doesn't matter what the supplemental witholding rate is. At the end of the year, income from an employer is all the same, and if the supplemental rate witheld is above the employee's marginal tax rate, the excess amount deducted will be refunded. If the supplemental rate witheld is lower than the employee's marginal tax rate, the employee will owe money on April 15th.
If one is presently employed elsewhere, or will be shortly after separation from Comair and they find the supplemental income witholding rate was higher than their marginal tax rate will be for the year, they can adjust the number of exemptions claimed on their W-4 to balance things without having to wait until April 15th to recover excess taxes taken.