I'm not saying the formula would nessicarily be that of a parabola (or that I know how to make such a formula for ENY).
I'm just saying that if you take a parabola (half an upside down parabola, actually) and place it over top of a graph, x axis being arena population, y axis being percentage needed for ENY to take effect, you get a general picture of what I'm talking about (the higher the numbers, the smaller the percentage needed before ENY starts to limit planes, and the lower the numbers, the larger the percentage needed).
Still trying to work out an exact formula, but not having much luck