When I started this discussion several years ago, my return, like that of most people, was was one where there was no difference between line 15 - taxable income, and Line 11 - adjusted gross income (AGI).
If however, one has to use the Schedule D Tax Worksheet, and/or the Qualified Dividend and Capital Gain Tax Worksheet, then the two would be different and Tax/AGI = Effective tax rate, as it does for mine. (I didn't have any AMT or Credits as you did.)
