I don't think anyone at a decent level uses a centre aim.
A couple of suggestions, first glasses. If you use shooting glasses have you got the focus set at the focal length of the foresights? If not this is key to keeping concentration on the sights. A bonus is if like me your vision is predominantly long sighted with the prescription set for the forsight you can only see the target as a fuzzy blur any way!
If you use an iris, Don't! As that helps you get sharper on the aiming mark.
If all above is correct, try a very low aiming point, I, and I know I'm not alone, as many top level shooters also use an aiming point close to the bottom of the card, the 2 or 3 ring. A true area aim. The problem with a direct sub 6 is you are trying to ballance the sight picture just under the aiming mark, and its easy to get sucked into looking at it.
Practice on a blank card, a reversed card, you may find you get tighter groups doing this than with an aiming mark, practice that with a very low aim. Using the very low aim elevation is easier to judge as you can see the bottom of the card if you drop to what would be a 7/8, it just takes a bit of practice and discipline to not be drawn up.
Good Shooting
Robin
Walther KK500 Alutec expert special - Barnard .223 "wilde" in a Walther KK500 Alutec stock, mmm...tasty!! - Keppeler 6 mmBR with Walther grip and wood! I may be a Walther-phile?