If you properly used TurboTax to amended your 2017 tax return to reflect that you had the contribution returned before the due date of your 2017 tax return, including extensions (by revisiting the Traditional and Roth IRA Contribution section and telling TurboTax that you made the contribution but had the excess returned), TurboTax will have properly corrected your Roth IRA contribution basis information on the IRA Information Worksheet in the amended file. It's this amended file that you should have used to transfer in when beginning your 2018 tax return. If you instead began your 2018 tax return by transferring in your original 2017 tax file, go through the Traditional and Roth IRA Contributions section in 2018 TurboTax and indicate that you want TurboTax to track your Roth IRA basis, then enter the corrected basis value, overwriting the incorrect value.
