When aiming a pistol, especially with open sights, you should be aiming at an area, not a point. Point aim is fine for a rifle because it can be held steady, but even a champion's pistol moves around.
The usual setup for the sights is so that there is a gap (size to suit the individual) between the black bull and the front sight. If you try to sit the bull on the sight, the two tend to blend together. It's known as "sub six".
The bull is also known as the "distraction bull" because you are supposed to be only looking at the front sight, not the target.
This is still probably the best setup for any target.
Try shooting a blank sheet of paper, area aiming somewhere below centre. You'll be surprised just how good your "scores" would be if a bull and scoring rings were superimposed.