This is really tricky. A torque wrench is of no use. The important thing is to place the nut in exactly the same position it came off.
The problem is that the pinion bearings are set up by crushing a special spacer between the bearing inner races till it takes a certain small torque to turn the tightened bearings only (totally unlike the way you do front wheel bearings). This original tightening is done with only the pinion installed in the case and the measurement can not be retested with the gear set in place.
So, what you have to do is retighten the nut to the exact point it was before removal. it should be tightened to a torque only if it won't go farther. if it will go farther then there is already wear on the pinion bearings and they are probably the reason for the original leak.
Bosch Master, ASE Master, L1
33 years MB technician