Form 8829, line 3 is the business square footage percentage.
You will have to go through the Sch. C flow.
Click "Edit" on the "Self-employment income and expenses" section
Click "Edit" on the business on the "Your 2017 self-employed work summary" screen
Scroll down to "Home office" and click "Edit"
Go through the flow until you get to the screen "How big is this home office?" these numbers make the business square footage percentage.
