I think peekay4's >55% is probably ~~close, but I am NOT sure. Balance tolerances of often extremely tight. Think about this... Your flight is a few minutes late departing. You know that the cabin is turned and the FAs are ready to board 50-500 pax, but they are not doing so. And you can see the ramp rats still loading bags/cargo into the forward hold. Ahem... There may be a darn good reason. To a far lesser degree, that's also one of several reasons that front cabin pax often board before the masses of unholy and unwashed. In some locations, weather (winds) and pre push back ice loading may play a part as well. Some carriers/load planners pay more attention than others. An unplanned, forceful WHOP to the rear end can do a lot of damage and you can bet that it will always delay the departure...
