As the other guys have already said, there's no torque figure for the castellated nut, most folks arrive at the setting by feel.
What I do is jack up the wheels so they rotate freely. Tighten the nut while spinning the wheel slowly with my other hand until I can start to feel drag. Back off the nut until a slot lines up with the split pin hole. There's supposed to be a bit of movement allowed in the bearings, it's not like a modern design where everything is rock solid, no play.
Having said that, I've no idea how much end float the bearings have because I've never used the dial gauge option but the wheel spins smoothly with no significant movement at the wheel rim that I can find by hand. It passes it's MoT test like that so I'm guessing they can't find anything wrong because they do pull you up if they think there's any sort of resistance (brakes binding) or bearing wear.
Brian