That's the purpose of Form 8606 Part I, to determine the ratio of basis to the overall amount in your traditional IRA and distribution and to calculate the taxable and nontaxable amounts based on that ratio.
After entering the Form 1099-R reporting your distribution(s), click the Continue button on the Your 1099-R Entries page and answer the questions in regard to your nondeductible contributions and your December 31, 2018 balance remaining in your traditional IRAs. TurboTax will use this to prepare Form 8606 to determine the taxable and nontaxable amounts of your RMD.
If any of your distribution was a Qualified Charitable Distribution, the QCD comes all from the pre-tax money in your IRAs so it is not included on Form 8606.
