Originally Posted by mechBgon

It doesn't matter where you put the magnet on the spoke. The magnet will still trigger the sensor at the same frequency when the wheel's turning, no matter where on the spoke you've placed the magnet. So don't sweat it

Originally Posted by Sheldon Brown

With one-magnet cyclecomputers, you usually have a choice of several places on the wheel to attach the spoke magnet, depending on the spoke pattern of the wheel. Generally, it is best to mount the magnet as close in toward the hub as possible. The closer in you mount it, the more slowly it will pass by the sensor, giving the sensor's magnetic switch more time to respond. If the magnet is too far out, the computer may give erratic readings at higher speeds.