It's more than just replacing the bead. A lot depends on your rear sight --- the MK I Ocular System that comes in two models, left eye and right eye.
If you put on a bigger bead, you will be aiming lower. If you put on a smaller bead, your aim will be higher. So, determine what range you're shooting at and pattern the gun.
If you're shooting too low or high, then you know what size bead to get.