Form 8606 has nothing to do with determining the amount of RMDs. Form 8606 Part I has only to do with determining the taxable amount of IRA distributions, regardless of the reason for taking a regular IRA distribution, and nothing to do with 401(k) distributions.
For the year that you reach age 70½ and subsequent years, RMDs are required from 401(k) accounts, except if you are still working for the employer and you are not a more-than-5% owner of the company. An RMD from a 401(k) is required to be satisfied from that 401(k), without regard to any other qualified retirement accounts. You are required to satisfy any RMD required of the 401(k) before rolling over any other amounts distributed from the 401(k).
Like money in a traditional IRA, RMD requirements make it certain that money in a 401(k) account will eventually be distributed. Nobody can live and work forever to maintain the still-working exception to RMDs from the 401(k). Even if the employee dies while still working, the employee's beneficiaries will have to take distributions from the 401(k).
