There's no one definitive answer to this question, because different scopes and mounting systems will require different levels of torque. But a good rule of thumb is to tighten the base screws to about 15-18in. lbs. This should provide a good, snug fit without overtightening and causing damage. If you're not sure how much torque to apply, I always find it best to err on the side of caution and start with a lower setting.