If the rooms in your home are all
about the same size, you can figure the business
percentage by dividing the number of rooms used in your business by the
total number of rooms in the house.
The most exact way to figure this proportion is to measure the square footage devoted to your home office and find what percentage it is of the total area of your home. If the office measures 150 square feet, for example, and the total area of the house is 1,200 square feet, your business percentage would be 12.5% (150 ÷ 1,200).
See The Home Office Deduction
https://turbotax.intuit.com/tax-tools/tax-tips/Small-Business-Taxes/The-Home-Office-Deduction/INF120...
