When shooting the "correct" NRA bullseye rifle target for the specific range, the black portion is proportioned so as to appear as wide as the front sight. Using the "6-O'clock" hold should give a precise, repeatable sight picture. If the black part of the target is not the correct size then the system won't work as intended. While some shooters can shoot accurately using the "center hold", a black front sight somewhere inside the black bullseye does not provide sufficient contrast for a precise sight picture and they shoot vertically elongated groups.
Often the 6'oclock hold can be used advantageously when shooting for groups using a low powered 'scope for the same reason. The black crosshairs can disappear inside the black but can be precisely aligned with the bottom-center of the bullseye (or top center too for that matter) Keep in mind that the black part of the target is an aid in aiming and sight adjustments put the bullet holes where they are needed. I use the standard NRA 100 and 200 yard rifle targets to zero my M1A at those yardages and most of my rifle students can pick up the gun and shoot bullseyes using the 6'oclock hold with a little explanation.