In Plan 401k to Roth 401k Basis

This tax year 2022 I did a partial in-plan 401k to Roth 401k conversion. Only pre-tax money was used for the partial conversion, but I do have after-tax money in the same 401k account.  I is there any cost basis that should be applied that will reduce my tax and if so, where is this entered in TT interview or forms ?