I just discovered that in 2015 I did not add my after tax 401K contributions when computing my basis for taxes on my Mandatory Required Distributions. How do I correct this error? Can I simply use the basis increment deltas that I already calculated on my form 8606s in 2015-2020, but change the actual basis carried forward to the value that would be the case if I started with the correct basis? This approach would mean I would still eventually recover all my basis but more slowly than I would have if I had correctly shown my basis in 2015.
