EN74 which covers fitting design and specification gives a torque figure of 50 Nm but that is for testing of brand new fittings only. A bit of dirt on the thread or a very oily fitting and unreliable calibration of the wrench will require different amounts of torque to get the fitting at the same tightness on the tube. I'm sure that someone here can give you more practical advice but for the time being try 50Nm. You can then put a spanner on it to see if you can get more movement, in which case you would increase the torque a little at a time until you can't get movement. Also compare how easy it is to undo the wrench tightened fitting with one that you have tightened by hand. You should be able to feel whether the wrench is set low or high.