generallee,
We are doing that right now, we are seeking a new type of fleet aircraft for a particular route structure. The manufacturers will tell you that the aircraft has no problem taking full payload on the selected route provided it departs to the north, they omit to tell you that you will leave passengers behind if you depart to the south.
So we will err on the conservative side and base assumptions on the less than optimum runway and a pretty hot temperature.
However, even on a 4 engine aircraft we wont base decisions on a double engine failure.
Mutt.