Assuming that you're not longer filing jointly, you'd make the computation using only your own earned income from 2019 (i.e., don't include your ex-spouse's earned income) in making the computation.
According to the 2020 Instructions for Schedule 8812:
You may be able to use your 2019 earned income on line 6a to figure your additional child tax credit. You can make this election if your 2019 earned income was greater than your 2020 earned income. Use the Earned Income Chart, later, twice—once with 2019 amounts and once with 2020 amounts to see if your 2019 earned income is greater than your 2020 earned income.
