" I filled out form 8606 for 2020 and reported the "correct" basis equal to the reported distribution,"
that's not permitted, you can't make up a basis that gives you the tax result you want.
if you enter your true basis, and your total value of all IRAs at year end, as required,
the ratio there will determine how much of your distribution is not taxable.
your true basis is from line 14 on your last filed Form 8606, plus any non-deductible contribution for 2020.
