The maximum distance both parts of the sensor can be from each other is 20 mm.
The both parts of the sensor have to be setup so they are level and aligned with each other.
The two parts of the sensor have to be aligned in three ways.
The pictures below will explain things by showing how not to have the two parts of the sensor aligned:
1) The gap between the two parts is minimal.
(Put another way, the gap in the middle of the sensor which constitutes the x-axis is minimal.)
The picture below shows the gap.
2) The two parts of the sensor are aligned from top to bottom part with each other.
(Put another way, the top and bottom part of the sensor are aligned via the y-axis.)
The picture below shows the two parts of the sensor as very unaligned from top to bottom since one part of the sensor is clearly above the other.
3) The two parts of the sensor are level with each other; essentially, one part of the sensor cannot be too far forward from the other.
(Put another way, the two parts of the sensor are level on the z-axis so one is not too far ahead of the other.)
The picture below shows the larger par of the sensor being much more forward than the other part.
