I do not know of any demand factors as conditions vary heavily. You would have to go back to the mechanical engineer who designed the setup as he would know how much things could have been oversized and could possibly figure out a demand factor to use that was acceptable to the AHJ. Otherwise the above method is correct.
It is my understanding that even if the power were out for a long time, the units would not come on at the same time due to random time delays built into typical units to avoid instant on with just a split second power return potentially damaging the the units.
