You may claim only what you actually spent.

IRS Publication 587, "business use of your home", (31 pages) has the instructions and an example of form 8829 where you claim mortgage interest, utilities, insurance, taxes, etc. Form 8829 type expenses are limited to your net profit from the business, in other words you can't use home expenses to create a loss. http://www.irs.gov/pub/irs-pdf/p587.pdf

Expenses not connected to the home are fully deductible on 1040 Sch-C and do not go on the form 8829. Examples would be expenses such as advertising, office supplies, business telephone, etc.