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Which tax computation worksheet are you looking at? There's no column d in these worksheets, AFAIK, assuming you're using ProConnect Tax (or even Lacerte). A bit more info and context will help.
Still an AllStar
That's the worksheet on page 74 of the forms instructions: https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
It's not a calculated amount; it's literally the number printed in the instructions. It's the true-up to take graduated brackets into account.