I had found this a couple days ago:
To compare your effective rate to Romney's, get your 2010 Form 1040 and divide line 60 (total tax) by line 37 (adjusted gross income).
There are other ways to measure effective tax rates.
Some say you should divide line 61 (total tax) by line 22 (total income), which typically results in a slightly lower effective rate. (Obama's drops to 25 percent.)
The Tax Policy Center uses an even broader measure of income that includes some items left out of adjusted gross income, such as tax-exempt interest and un-taxed Social Security benefits.
By this measure, the effective tax rate is 9 percent for all individuals and 14.7 percent for those in the $200,000 to $500,000 bracket. The rate is higher for people in higher brackets and lower for those in lower brackets.
Read more:
http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2012/01/18/BUD91MR4RS.DTL#ixzz1kOqLm74G