I tried the value that Kymco advises in the service manual, but I found it to be uncomfortably tight...
Since I do variator maintenance and tuning more often than Kymco had in mind when writing the manual - and stripping the threads off the crankshaft would mean a lot of work and quite a lot of money for a new crankshaft - I decided not to go with Kymco's specs.
I apply a thread locker (Loctite 243 retainer) to the nut, and tighten it only as much as I feel comfortable with - tight enough to keep the ramp plate from slipping, but not so tight that you can feel things bending or galling...
Never had a variator nut coming off - works for me, but YMMV of course...