That centre nut (not sure if it's the "locking nut" you're referring to) needs to go to 186nM. So you do need a proper torque wrench to do this. You also need a proper Ducati "clutch holding/locking" tool. Although I believe you can build them yourself, I bought mine for approx. £70 from Carbon Traders, delivired next day in an emergency. The centre nut requires a 32mm sprocket, so make sure you have a big enough one.
Here's what happens if you don't tighten the centre nut to 186nM:
http://kapitalmoto.co.uk/node/27 and
http://kapitalmoto.co.uk/node/28
Threaded Mode