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As with anything it's critical that they are mounted level to the rifle. If you torque one side down too soon they will be out of level. Sneak up on "tight" and you might even use feeler gauges between the ring halves to ensure that they are gapped evenly.
I use this one from Defensive Edge. It's a level and Angle Cosine Indicator all in one and replaces the top half of a Nightforce scope ring.