The calculation you need to make is based on Credit Limit Worksheet B in the 2021 Instructions for Schedule 8812. It’s a very complex process.
If you’ll notice the asterisked text at the bottom of the Credit Limit Worksheet — Line 15 box in the 2021 Instructions for Form 8839, it notes:
* If you are directed to complete Form 8839 by Credit Limit Worksheet B in the Instructions for Schedule 8812 (Form 1040), include the amount from line 14 of Credit Limit Worksheet B instead of the amount from Form 1040, 1040-SR, or 1040-NR, line 19.
Based on your statement that you “weren't physically living in the U.S. for over six months,” Credit Limit Worksheet B for Form 8839 would apply to you, since you presumably wouldn’t have checked a box on line 13 of 2021 Schedule 8812 (Form 1040). (You would check one of those boxes if you (or your spouse if married filing jointly) had a principal place of abode in the United States for more than half of 2021 or you (or your spouse if married filing jointly) were a bona fide resident of Puerto Rico for 2021.)
Once you’ve completed Worksheet B, you’d plug line 15 of that worksheet into line 4 of Credit Limit Worksheet A, and this will result in the amount you’re seeing in TurboTax.
