Bigger wrenches is the usual cure. But, I NEVER tighten anything using a formula. Hand tight and a half turn with a wrench may work for 50% of joints, but I can guarantee that it will NOT tighten all of them. My procedure with angle stops is,,
1. remove from the box.
2. snug the nut against the sleeve so it stays in place.
3. slide the valve onto the copper.
4. Tighten the nut using two wrenches.
5. Keep tightening it until you are sure it will not leak, usually when it will NOT turn any further.