According to the campaign, Romney earned $13,696,951 in 2011, of which he paid $1,935,708 in taxes.
But those figures include $1,770,772 in charitable contributions that he could have deducted, but chose not to. Since the campaign acknowledges that nearly all of Romney's income came from investments, we assume he paid a 15 percent rate on that $1,770,772 -- or $265,615.80 in taxes that he didn't have to pay.
If you subtract that figure from his total taxes, you're left with $1,670,092.20 -- the amount of tax he would have paid if he'd deducted all of his charitable contributions.
And that's 12.2 percent of his overall income.
Romney's official return reveals that some of his income was subject to the Alternative Minimum Tax at a maximum rate of 28 percent. If the contributions he didn't deduct were taxed at that rate, it means he could've reduced his effective tax rate even lower -- to 10.5 percent.