The rule-of-thumb (no pun intended) I go by is to tighten the nut until the QR lever starts meeting resistance when it is parallel to the axle. When it's in the closed position and perpendicular to the axle, there should be enough clamping force to keep everything in place. The amount of reistance should be enough that it leaves a small temporary impression in your thumb upon closing but not so much that you can't get the lever totally perpendicular to the axle.