The bottom line is that it doesn't matter where the magnet or the sensor are placed, so long as they cross one another, and the spark occurs, at about 28 degrees before top dead center. The sensor can be placed any where convenient, and the spot on the hub for the magnet then chosen so that the timing will be right. Most sensor mounts then provide for a degree of fine adjustment to get the timing fine tuned just right to 28 degrees. Check your system to find out exactly when the spark occurs when the magnet crosses the sensor. Usually it happens just as the magnet is leaving the sensor after having crossed it.

AV8TOR