Another approach is to create a bmp file with a small dot or rectangle. Insert white space on the left side (moving the shape right) and print. Repeat these steps until the shape is on the right edge of the receipt. Now you have the exact width, and can center your logo.
Note that you're not just restricted to 1 bitmap, so you can great headers, footers, custom seperators, etc. Our receipt has a logo on top and references to twitter and facebook on the bottom. I'll try to post a photo if there's any interest.