If I have roomates, do I divide the number of rooms in the house by the number of roomates I have for the purposes of business-as-home expenses?

For example, if I rent a house with three other people, I'm not paying to rent the entire house, just like I'm not paying the entire heating bill. When calculating what % of my heating, electricity, and rent payments I can deduct, how do I factor in the fact that I'm sharing the home with 3 others?