Torque of the rails or torque of the post?
If the max. torque on the rails is 6Nm then you shouldn't exceed that in no case. The 12-15Nm is the maximum when the clamp is used with metal rails.
In your case, start by taking the clamp apart and clean the bits that touch the rails of your saddle from any remains of grease with a solvent. Check the clamps for sharp edges and give them a sanding with fine paper if required. Then apply some carbon mounting paste, grease the bolts of the clamp and tighten to the recommended torque for the saddle.