See https://www.irs.gov/publications/p946#en_US_2020_publink1000107386
We have been here before and I do not believe Form 3115 is going to be applicable when the wrong basis is used. That would not be an impermissible method of accounting, but more of a math or posting error.
See https://www.irs.gov/publications/p946#en_US_2020_publink1000107386
@AmeliesUncle
