Have you been watching me Doc? I'm not called Mr. Safety for nothing.
Andrew, I had to get crestive with my installation, because the magnet is designed to be used with spokes, not the wheel of an ATV. The magnet on my particular setup was screwed onto an apparatus that would normally be clamped to the spokes, so I unscrewed it and measured the thread. I then drilled and tapped a hole in the rear sprocket and screwed the magnet in with some Loctite.
I placed the pickup on the swingarm and secured it with zip ties. I didn't use any kind of measurement, I just placed the pickup as close to the magnet as possible. The instructions indicated that I could either measure the circumference of the wheel or roll it forward for one full revolution. Since the tires have big lugs on them, I chose to roll it forward. The cycle computer will accept a huge range of wheel diameters, so mine was no problem.
I wish I would have purchased a backlit version.