In my situation, line 16 is divided by line 11, representing Tax divided by Total Income. Total income encompasses items such as IRA contributions, which are certainly part of your total income, although taxes are deferred.
Example: Billbob's total income was $50,000 and he made a $5,000 IRA contribution. His adjusted income is $45,000 (see line 10). So when calculating the effective rate, it seems reasonable that he would include his IRA contribution as part of his earnings to calculate the effective rate, or $50k in this example, not $45k. Using the lower value (45k) yields a higher effective rate as per this post's original calculation. (See line 10 on the 1040 form and schedule 1).
FWIW, in my case line 24, total tax, is a value derived from subtracting a tax credit from line 16 and adding a self-employment tax, yielding a higher value. I am not sure why these values preclude TT from using line 24 as the denominator. That's for another day.
