The calculations are done on Form 8880 itself, but if the resulting credit is zero, TurboTax doesn't include Form 8880 in your tax file. Because this credit is a nonrefundable credit, you don't get any benefit from the credit if your income tax liability is already zero without this credit. This credit cannot be used to reduce self-employment taxes. Because this credit is meant to benefit those with lower incomes, it's common that those otherwise eligible for the credit have no income tax liability against which to apply the credit.
