I was divorced in 2021 and I need to determine how much of our combined 2020 AGI was mine. How do I determine this?

If you filed your 2020 tax return as Married Filing Jointly then you enter the 2020 AGI as reported on the 2020 federal tax return Form 1040 Line 11.  You do NOT adjust the 2020 AGI when e-filing the 2021 tax return.

If you filed a Joint return in 2020, then use the full amount of your 2020 AGI from your joint return.  Do not split it.