Hi crj705,
For instance, is there a way to quantify how much of a margin you have between the assumed performance numbers and the actual temp when doing a flex takeoff?
The engines would be producing the equivalent thrust if the actual conditions were at the assumed temperature. The aircraft is however flying in air which is denser than the assumed temperature, so it will perform much better than if the density conditions actually existed.
I would guess the improvement margin is proportional to the density difference (between actual and assumed temperatures in degrees absolute).
So If I used a flex of 70 degs C, when the actual conditions were 20 degs C, then my performance would be better by a factor proportional to (70-20)/(273+70).