When renting out a part of your primary residence, there is one screen that will ask you the "WRONG" question for your specific and explicit scenario. The question reads: "Percentage of time I used this item for this business in 2020 (e.g.; 80%)"
What you need to enter in that block is the percentage of floor space, not the percentage of time. The program already knows the percentage of time, based on the "in service" date you entered on a previous screen.