I assume that you did this Roth conversion in 2020. With a total of $8,550 of nondeductible traditional IRA contributions, as long as you have no other traditional IRA accounts at the end of 2020 (do not make the mistake of rolling the 401(k) into an IRA before the end of 2020), the nontaxable amount will be $8,550 with the remainder being taxable on your 2020 tax return. Your 401(k) is not an IRA, so it doesn't factor into the calculation.
