If you used TurboTax to prepare your 2017 tax return, TurboTax should have acquired as conversion basis the amount that was rolled into the Roth IRA. This should be present on 2017 [not 2018 as I originally typed] Form 8606 line 24 if line 23 is nonzero. If it's not there, click the Continue button on the Your 1099-R Entries page and proceed to where TurboTax asks if you made any Roth conversions, the enter the taxable and nontaxable amounts that came from the traditional 401(k). (Perhaps it was all nontaxable if you sent the taxable portion, if any, to a traditional IRA.)
