how to find the "what-if spreadsheet"?

The What-If worksheet is only available in the Desktop program.

Go to Forms Mode, click Forms in the upper right or on the left for Mac. Then click Open Forms box in the top of the column on the left. Open the US listing of forms and towards the bottom find the What-if worksheet. It's right under Estimated Taxes.

Or try…Go into Forms View. Once there, at the top of the left column, click on the icon for "Open Form". A popup window will appear. In the text line, type the word "what" without quotes. You should see "what-if worksheet" appear as a selection choice. Double-click it to launch the form.

Do you need a screen shot?

The What-If worksheet is only available when using the TurboTax desktop editions in Forms mode.

Click on Forms.  When in Forms mode click on Open Form in the upper left of the screen.  Type in What-If.  Click on the result to open the worksheet.