There is some confusion to the FN SCAR rear sight. Hopefully this will help clear things up.
The windage is 1 MOA clicks. There are 36 clicks of adjustment for windage, so 18 from one end will put you in the middle.
Elevation adjust is 1.5 MOA per click. There should be enough adjustment to go below the 200m mark and have a 100m zero, and plenty of adjustment past the 600m mark.
Ideally the rifle should be zeroed with the front sight, and the rear sight left mechanically zeroed and only adjusted for wind or distance.