Just a small amount of tension on the screws is needed. The screws should keep the endbell seated in position until water pressure holds the endbell itself. Pre-tighten all screws evenly using only two fingers on the hex driver until endbell is well-seated. Then apply a final torque on all screws by turning them 30-45 degrees more.
DO NOT EXCEED 45 degrees, since deformation of threads in the plastic housing could be the result.

«Two-finger» tightening of screws. Indicator arrow shows position before final tightening.

Final tightening 30-45 degrees from «two-finger» tightening.