@Don-TT
Your understanding of the What-If Worksheet is correct, and you seem to be using it correctly. What specific boxes can you not type into? The calculated values probably come from something higher up in the form, possibly one of the "Smart Worksheets." Itemized deductions come from the Itemized Deductions Summary" all the way at the bottom of the form.
The danger in using actual overrides in forms mode is that an override can break the internal calculations. The flow of the calculations is not always obvious, and an override can sometimes unexpectedly affect something that does not seem to have any connection with what you overrode. You might not realize that you are getting wrong results. If you use an actual override, the amount will be shown in red in forms mode.
