Each time the magnet passes the sensor, it registers one revolution. As you have allready put in the circumference of your tyre, it wouldnt matter where along the spokes the magnet is set up, as the circumference of your wheel registered in the computer is the same. Near the hub makes sense to reduce rotational weight, and keep dust and dirt off it, but sometimes it's not possible if the spokes are too far away from the fork. Doesn't really matter though. I actually run mine close to the tyre, so i have less cable running over my bike, looks tidyier.