Firstly you will never get that high a bias in such a large number of people.
Secondly if the flights are known to carry large people on a regular basis the standard weight is increased. Flights in NZ to certain islands use a higher standard weight due to the average size of the passengers being higher.
The issue is really mining charter in turboprops where you will get 30 odd guys who are all well above the standard weight yet it seems not to be a problem. If the book says to use standard weights that's what you use unless the ops manual has something else in there.