A lot of guys with an A2 front sight prefer the lower 1/3 which keeps the dot above the post when looking through the center of the tube and a less obstructed view through the tube, particularly with the small micro tubes.

I rarely have my folding sights flipped up, but I prefer the lower 1/3 because it gives me a little more room to work with cheekweld that I generally find cramped wearing hearing protection.