Normally, I have found that if you answer the 1099-R questions correctly, it handles the backdoor Roth conversion correctly. But here is a situation that happened to me and I had to pay tax but it is correct.
Up until 2019 I had not value in an IRA as I would do a Roth Conversion the next day of contributing to the IRA and used to do it in the year i.e. between Jan-Dec. Last year I did the same. But then mid year, I retired and I rolled over my 401K to an IRA. Now I had an IRA position at the end of 2020 and also a Roth conversion of 7K. So TT calculated the ratio of the 7K that was taxable and most of it was.
If you had a basis at the end of 2020 of all your IRAs combined then the back door would be partially taxable.
Brs,
