You need to look into this guidance from the IRS:
Requirements to Claim the Home Office Deduction
Regardless of the method chosen, there are two basic requirements for your home to qualify as a deduction:
- Regular and exclusive use.
- Principal place of your business.
Regular and Exclusive Use.
You must regularly use part of your home exclusively for conducting business. For example, if you use an extra room to run your business, you can take a home office deduction for that extra room.
This means you cannot use this area of your home for any other purpose than business use. You shouldn't even be entering it except for business purposes (if the room connects two rooms, the IRS will expect that you to prove that you do not regularly pass through your office). As such, it's unlikely that your home office would take up 50% of your apartment. As @Dilip Sarwate pointed out, this would likely trigger an audit. You need to be careful in determining whether your home workspace constitutes a home office by the IRS' standards.
You've got it just right. Multiply the square footage of your office by 5 to get the correct dollar value. Put this amount in Line 30 of your Schedule C.
You'll need to fill out Form 8829 to support your declaration of expenses in Line 30 of your Schedule C. This will ask you to provide the full expense for the apartment and then require you to multiply it by the ratio of square footage. In your example, the home office is 50% of the total area. You would multiply all expenses by half to get your total business expenses.