The NDS values are derived basically by taking the lower 5th percentile of the breaking strengths of each species and then dividing that by 2.8 (this of course, depends on the strength characteristic being tested, but is true for deriving bending strength allowable numbers). I could explain the 2.8 in more detail if you like, but that is the number that is used. I believe many of the numbers reported in the Wood Design Handbook are average values so they would need to be reduced by more then the result of dividing by 2.8.