If you want to be absolutely certain I'd ask Thorn or Rohloff.
But, IMO, it isn't necessary to know, I'd just snug it up fairly tight. I haven't plucked the opinion out of thin air, here are the things that I've considered:
The Rohloff manual not giving a figure in the appendix, where it does for those that require it
There's only a short thread on the axle, it may be that limit is fine
The nut presses against a spring, with a washer between, so it's not pressing against anything that's requires a fixed pressure
Lastly - it's called a lock nut and these are by nature what they say, they're there to hold things in place rather than to load something.