Even when it's working as intended by the the developers, the Tax History display of Tax bracket % is pretty useless. TurboTax's calculation of Tax bracket % assumes that all of your taxable income is taxable as ordinary income, even when it's not. What you really want to know is marginal tax rate, something that TurboTax is incapable of providing except by the user adding various types of income and manually calculating the marginal tax rate as change in tax liability divided by change in income. Manually calculating marginal tax rate will also account for changes in deductions, tax credits and the taxable amount of Social Security income that result from the change in a particular item of income, things that can have a multiplying effect on tax-bracket rate.
