For a single filer, what's the maximum total income to make sure you'll pay zero tax on the conversion?
Depends on your tax situation.
Typically it's going to be the standard deduction, which for 2020 filing single is $12,400.
Of course, if you had a job that earned $3,000, you'd have to subtract that out, so only $9,400.
And then it's possible, with refundable credits such as the CTC, APTC, AOTC, and the EITC, that you could convert more and still pay zero tax at the federal level.
Finally, there are other weird things that could happen, like the foreign tax credit, or QBI, or loss carryforwards, that would affect the number as well.