I use a short plastic ruler and place it against the face of the stem with the numbers facing up. You can use a tape measure or whatever you have. I make sure one of the "tick marks" is directly in the center (it's easy to align that using the clamp bolts). Then I look down and make sure the tick mark is lined up with the center of the tire. It's easier than it may sound. If you use that method you might be suprised at how much it's off center.