Review chapter 4 in publication 463.
https://www.irs.gov/forms-pubs/about-publication-463
The key question is where is your principal place of business. Mileage is calculated one way if you have no main place of work, and a different way if you main place of work is your home. Note that you don't actually have to declare a home office deduction for your main place of work to be your home, but you have to meet the same basic tests.
