Figures released by independent U.S. Senate candidate Angus King showing he paid average federal taxes of 22.2 percent over the last seven years would be different if he’d calculated it the same way as Republican presidential candidate Mitt Romney.

Republicans on Tuesday accused King, a self-made millionaire, of using a methodology that understated his income to make it seem as though he was paying a greater share of taxes.

King computed his rate based on taxable income “after deductions and loopholes, artificially lowering his gross income and effectively raising his perceived tax rate,” David Sorensen, spokesman for the Maine Republican Party, wrote in a statement.

Using Romney’s methodology, King paid an average of 15 percent, less than many middle class Americans.

King’s accountant, Scott Small, said he based his calculation on taxable income after personal exemptions and itemized deductions, which he described as the standard among accountants for establishing the effective tax rate. Romney based his tax rate on adjusted gross income.

There’s no clear answer on which is right.

Steven Colburn, accounting professor at the University of Maine, said the textbook his students use defines effective tax rate as total taxes divided by total income, a standard Romney came closer to meeting.

But Thomas Cooke, a professor of business law at the McDonough School of Business at Georgetown University, said the King campaign did it the “proper way” with its methodology.

Bob Williams at the nonpartisan Tax Policy Center in Washington said the best standard is total income. Barring that, the best measure of income on a tax return is the adjusted gross income used by Romney.

“What King did is not right. What King did is overstate his taxes, potentially by a lot,” Williams said.

Democrat Cynthia Dill called on the three leading candidates to release their tax returns, and King obliged Monday by releasing seven years’ worth, amounting to hundreds of pages of tax documents.

That created a scramble in which Dill and Republican Charlie Summers released their documents Monday night.

All told, King averaged more than $500,000 in annual income using adjusted gross income over the seven-yearperiod. Summers and his wife averaged nearly $91,000 in annual income over eight years of federal tax returns. Dill and her husband reported about $65,000 in average annual income over 10 years.

At a recent debate, King suggested his taxes should be higher, advocating for bringing the capital gains tax in line with wages. Over seven years, he averaged $231,000 in earnings from capital gains — profits from sale of stocks or property — which are taxed at 15 percent.

King, for his part, insisted he was trying to do the right thing in releasing his tax returns and in his analysis.

And he said there was a reason his taxable income is so much lower than his gross income: It was because he and his wife averaged $75,952 a year in charitable contributions, which can be deducted on federal income tax returns. “If we didn’t do those charitable donations, it would be higher,” King said.

Three other independents in the race — Steve Woods of Yarmouth, Danny Dalton of Brunswick and Andrew Ian Dodge of Harpswell — were not included in Dill’s proposal.