Better to buy this. Swinging arm bearing tool
Then you can keep the 12 mm Hex Nut in place when fastening the big nut to the correct torque value.
I have one and it works great.
And finally pulled the bevel box off the swing arm. Input shaft feels smooth but stiff. Paralever bearings were pretty stiff with old filthy grease and the pivot pins are a little worn.
All off to the garage to have new bearings to be fitted.