Does President Obama Pay a Lower Tax Rate than His Secretary?

From White House Dossier:

President Obama paid a total federal tax rate of 20.5 percent on a gross adjusted income $789,674, a rate that may come in below that of his secretary.

Obama has spent the past week touting the Buffett Rule, which calls on those who make $1 million – just a little more than Obama made – to pay at federal tax rate of at least 30 percent. The rule was inspired by Buffett’s comment that he paid a lower tax rate than his secretary.

The most recent information about salary regarding Obama’s secretary is for his former secretary, Katie Johnson, who is listed by the White House as having made $90,000 in 2010.

According to Wikipedia, Johnson is 31 years old and now attends Harvard Law School. I don’t know about her personal life or what her deductions would be, so I can’t assume any children or deductions.
On a $90,000 salary, she would pay $16,578 in federal taxes, $3,780 to Social Security, and $1,305 in Medicare taxes.

That adds up to a total federal tax burden of $21,663 on $90,000 in adjusted gross income, or a tax rate of 24 percent, well above Obama’s rate of 20.5 percent, even though Obama’s 2011 salary was nearly nine times the 2010 salary of his secretary.