Just follow the screen instructions and let the program do the math ...
A special “lookback” rule in the Dec. 2020 stimulus bill lets taxpayers claim this valuable credit – even if your income was higher in 2020 and 2021.
If your 2019 earned income is higher than your 2020 earned income, and results in a higher credit, the lookback allows you to use 2019 earned income to calculate the 2020 earned income credit.
Taxpayers can use their 2019 earned income to calculate their credit for tax year 2021 if your earned income in 2021 is less than it was in 2019 and using the lookback results in a higher credit. This bears no effect on your calculation of gross income. The IRS will have math error authority to correct an error if one is made under this section.
