I've been bolting a 19" spare on my rear door carrier for 15 years now and never, ever used more than 50-60 ft/lbs max and it's never loosened or fallen off yet. Remember there are no road pressures or stresses on the spare wheel unless someone sits on it eating their lunch

PS: Remember some joker has to remove the spare sometime at the roadside with a normal wrench and it may be the wives. Using 103 ft/lbs torque whoever removes the spare needs a 3' breaker bar or Herculean muscles.