Thanks! In Turbotax, I chose "add supporting details" for the "A Overpayment, if any, as shown on the original return or as previously adjusted by the IRS (not including penalties)" under "Original 2020 Return Overpayment Smart Worksheet" section on the 1040-X form, right before line 18. I put my original refund plus the UCE refund and it totaled them up and carried it to line 18. Now my new (3rd) refund on line 22 is correctly lowered to $900.
(The "or as previously adjusted by the IRS" is the key detail I missed.) Thanks again for the help!
