Accurately determining marginal rate

I don't think the Tax bracket % shown on the Five Year Tax History report is quite correct. It says that it is based on Taxable Income, which includes dollars potentially taxed at a lower rate (e.g., capital gains). It is showing 32%. However, if I subtract $100 from an ordinary income entry, the total tax decreases by $24, indicating the "tax on the last dollar" is 24%, not 32%.

 

For the purposes on fine-tuning how much of a distribution to convert to Roth and how much to put back into IRA, isn't my calculation the more accurate?