Will Turbotax allow a "what if" scenario for determining 2018 taxes. For example, if I roll money from a traditional IRA to a Roth IRA, what will be the consequences for moving various amounts of money.
The CD/download version of 2017 TurboTax implements most of the relevant tax changes for 2018 on its What-If Worksheet available in forms mode. (Mark the Use 2018 Tax Rates box.) However, it does not calculate the Qualified Business Income Deduction, that's left to the user to calculate and enter.
Enter on line 15b the taxable amount of your Roth conversion.