Use the total square footage of all space that is rented out "exclusive to the renter". Do not list each room as a separate rental. It's an absolute nightmare to do it that way.
So the rental use percentage will be that percentage of square footage of your total floor space that is exclusive to the renter(s).
If your house is let's say, 2,500 sq feet with four bedrooms and the 3 bedrooms being rented out add up to 1,000 square feet, then 1000/2500=0.4 or 40% of your floor space that is being rented out.
As a four bedroom house I also expect it has more than one bathroom. So you can include in that the area of any bathrooms that are exclusive to the renters.
A bath that is available to the renters "and" you or "your" guests is not exclusive to the renter.
A bath that is shared by 2 renters but is "not" available to you is still exclusive to the renters.
