To make a long story short and summarize a lot of work, the tax planner is not calculating the correct amount of tax in my planning case (2023 Married Filing Jointly). The 32% marginal rate should be applied to additional taxable income over $364,200. My base case including a Roth conversion yields Taxable Income of $364,200, income tax of $72,601, Other Taxes of $1,140, total tax of $73,741. I have no reason to suspect any errors, all seems correct. However as I look at larger Roth conversions I can add up to $17,763 in other income and Quicken continues to calculate the additional tax at 24%. For example, in a planning case with a $17,609 larger Roth conversion the Tax Planner yields a taxable income of $381,809, which is correct. However the income tax calculated is $76,827, Other Taxes of $1,140, total tax of $77,967. Thus the incremental tax on $17,609 more income is $4,446, which is 24%. The incremental tax should be calculated at 32%, which would yield an additional tax of $5,635. Finally, at additional conversions of $17,764 and more, the rate rises above 24% and the correct 32% is applied to Taxable Income above $382,055. Whatever algorithm they have programmed does not correctly reflect the IRS brackets - instead of immediately going from 24% to 32% at the bracket boundary above $364,200, it is 24% at $381,963 and increases to the full 32% above $382,055. I wouldn't be bothered by this strange transition if it occurred near where the actual bracket is supposed to change at $364,200.
