I'm not an expert, but I believe most people do apportion other house expenses such as insurance, heat, electricity,etc. I don't think you necessarily need to use square footage as the basis for everything. If, for example, you need internet access for your home office work and you regularly work there 50 hours/per week. You also enjoy the benefits of internet access for personal use each evening and on weekends, but don't use it when you're asleep, cooking, out of the house, etc. You may decide that 50% to 60% of your internet bill should go to the office. You just need to be reasonable and able to back it up if you're ever audited.
