What torque measurement do I need to tighten my rear shaft nut up to?

I need to replace the rear brake disks. when I the wheel off there is a nut on the outside of the brake disks. Its no problem taking them off, but when I put them back on I to tighten them up to a certain torque measurement. However, the problem is I don't know what measurement I have to tighten them upto. If anyone knows I would much appriciate it.

I have a renault megane 1.9 dci on an 03 plate.

Many Thanks.