I agree with those posters maintaining that 40Nm is much too tight. I use 5-10 Nm routinely and have never had a lockring loosen. I find that if I've had a rear wheel worked on by a mechanic, the next time I go to loosen the lockring, I need to fasten the cassette tool onto the lockring with a skewer to allow me to wield enough force to crack the lockring. 40Nm seems just so unnecessary with steel freehubs. (My comments are not necessarily applicable to alloy freehubs, as I've never used one.)