Your traditional IRA has no bearing on determining the taxable amount of your In-plan Roth Rollover. The 401(k) plan administrator is required to determine the taxable amount of the IRR will be reported by the 401(k) plan in box 2a of the code G Form 1099-R that they issue reporting the IRR. If less than the full amount of your after-tax sub-account in the 401(k) was rolled over to the designated Roth account, the plan administrator will do a pro-rata calculation of the taxable amount based on your after-tax basis in this sub-account and the total value of this sub-account.
