You should be able to get your answers reading pub 583, Business Use of Your Home. IRS has guidelines for you to follow. Hope this helps.
Click on link: http://www.irs.gov/publications/p583/ar02.html#d0e1506
In addition to that, you might want to read, Publication 587 (2006), Business Use of Your Home
If you want direct answers (pub 587)
To find the business percentage, compare the size of the part of your home that you use for business to your whole house. Use the resulting percentage to figure the business part of the expenses for operating your entire home.
You can use any reasonable method to determine the business percentage. The following are two commonly used methods for figuring the percentage.
Divide the area (length multiplied by the width) used for business by the total area of your home.
If the rooms in your home are all about the same size, you can divide the number of rooms used for business by the total number of rooms in your home.
Your office is 240 square feet (12 feet × 20 feet).
Your home is 1,200 square feet.
Your office is 20% (240 ÷ 1,200) of the total area of your home.
Your business percentage is 20%.
You use one room in your home for business.
Your home has 10 rooms, all about equal size.
Your office is 10% (1 ÷ 10) of the total area of your home.
Your business percentage is 10%.