You would first determine the Fair Market Value of the computer (eBay, Craigslist, etc.) when you first used it for your business.
Then you can depreciate the business percentage of that Fair Market Value.
Unfortunately, the value of computers drop quickly, so the Fair Market Value may be fairly low.
Also, a computer is "Listed Property". That basically means you need to keep proof of the business usage, such as time logs of when it was used for business versus used for personal usage.
