Percentile function

[GREEZEE]

Here's what I'm trying to do. I'm trying to create a function that will give me the value that is in the nth Percentile given a list of numbers. I'm pretty sure this just boils down to me not being able to figure out the syntax within javascript, but I've searched, and just can't quite seem to find what I need.

The concept that I want to do is to draw dynamic envelope lines based upon a percentile of the deviation from a moving average.

Can somebody help me with the code so I can put it into an efs?

Thanks