From Publication 587 (2024) Business Use of Your Home
Figuring Your Deduction: Using Actual Expenses; “Business Expenses”
"To find the business percentage, compare the size of the part of your home that you use for business to your whole house. Use the resulting percentage to figure the business part of the expenses for operating your entire home.
You can use any reasonable method to determine the business percentage. The following are two commonly used methods for figuring the percentage.
Divide the area (length multiplied by the width) used for business by the total area of your home...."
It's my understanding that the IRS applies the same principles of space allocation to rental spaces, and explicitly allows for proportional allocation of shared spaces in business use, and rental property deductions follow the same logic in Pub 527.
So, I included 100% of the square footage of the rented room + 50% of the shared space SF; then divided by the total SF of the apartment.
