Torque is usually set by the bolt/thread interface which would be the clamp. The seat rails care about the clamping pressure, which you can't really measure. Ideally, both should have a specification. The saddle would have a maximum clamping pressure, which clamp area would then dictate a torque value. You would then tighten to the lower value.

Of course, the reality is the saddle rail is probably over-built around a worst case, and the seat clamp is probably conservative. Basically, you pick a number a tighten to it and hope nothing breaks.