If you will be taking an RMD in 2018, you will have a need to file 2018 Form 8606 to calculate the taxable and nontaxable portions of the distribution. If the 2018 RMD is your first distribution after your IRA acquired basis from the 401(k), it's this 2018 Form 8606 that should include the adjustment to the basis on line 2. Your tax return should also include an explanation statement for the adjustment to your IRA basis.
