All absolute and lower 1/3rd really means is the height of your optic mount. Some people, especially those with a fixed front sights, like a less cluttered FOV so they opt for a higher optic mount. This places the front sight post in the lower 1/3rd of the optic's tube. Some people like the center of their optic tube to be exactly where their irons are. Those are absolute cowitness and the front sight post will appear in the center of the tube. For those with flip down fronts (like you) it's just a matter of preference on how high you want your optic. Example (the left is absolute cowitness):
If you were to look through your irons on the lower 1/3rd, your dot would appear inside your rear aperture and be superimposed over the tip of your front sight post (assuming they were zeroed at the same distance as your RDS).