I'm trying to create a hypothetical: I want to know how much I may convert from a trad. ira to a roth and how it will affect my bracket and tax liability; how to in TTHB?

OK, you need to go the Forms in the upper right side (if windows computer), On a Mac it is View then Go to Forms. 

The click on Open Form if the left side of the screen.

Type in "What-if worksheet" and open that Form.

Click on "Copy Column first box, 

then go to the column, and click on "Check box to use 2018 tax rates,

then add to income the amount you are thinking about converting after you know what the tax was first on the return. 

add this amount to line 15b. 

Works very well especially if income in 2018 is about same as 2017