How to do percentile calculation in webi 4.1

I am stuck in this for over a week and I need to get over this. I have a measure "TAT" that shows the turn around times of over 10000 applications, but now I want to apply aggregate function(average) on top 90% of the values only(which would give me the average of least TAT). Can anyone help me out on this. I have tried ranking but that enables me to have that only with a dimension along with it such as application id. Is there any way by which I can have the required average in another variable, so that I can easily use it. Thanks in advance