Work from home and have specific place as a home office,I do not own the business, do I put my AGI from W2 as business income when calculating deductions for home office?

My job is out of another state and I am fully employed by them but have my office out of my owned home and a company car.  I am trying to do my itemized deductions for the home office and found that it asks for total business income.  I am not sure if this applies to my entire AGI for the year as I do all of my business out of my office but i do not own the business that employs me.