why don't you know the exact amount? how do you expect the IRS not to change your returns if you apply the incorrect amount? you can get transcripts of prior year returns using the IRS website so you will be able to find the correct CLCO.
https://www.irs.gov/individuals/get-transcript
you have to amend 2019 to claim the CLCO - skipping years is not allowed. once you know the CLCO from 2019, you can amend 2020
