Did you enter the Form 1099-R that reported the rollover from the 401(k) to the Roth IRA? You must do so. After entering the code G or code 1 Form 1099-R for the distribution from the traditional 401(k) and indicating that it was converted to a Roth IRA, TurboTax will include this amount in your basis in nontaxable Roth conversions to be subtracted on your Form 8606 Part Part III according to the ordering rules for Roth IRA distributions.
You can see that your basis in nontaxable Roth conversions is properly recorded in TurboTax by examining line 36 the IRA Information Worksheet.
