When you are in Forms mode, right click on the number in question and then select QuickZoom. This will take you to the form where the number is calculated. Or, you may be able to select Data Source if the number was input instead of calculated on a different form.
See the picture below for an example of the menu you will see.
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FYI, this trick no longer works as of 2021 while I am filing my taxes with Turbotax 2020 desktop. The right click does not open a menu. There is no way to find the source of obscure calculated fields. This is a painful problem because some tax information tells you where to enter data on say Form 4797 or Schedule D and TurboTax will not except data entry on either form.