The position of the speed sensors at the fork as well as the spoke magnets is not important for the speed measurement. So the mounting of sensor is variable as long as the radio range of 70-80 cm for the analogue devices (macrofree, macrohigh free und mini_pcfree) will not be exceed. For the digital devices the pitch between sensor and magnet must be maximum 4 mm.
The sensor only measures the amount of wheel rotation. In combination with the wheel circumference the device then calculates the speed.