How to measure level/depth of perturbation

Hi, I'm trying to simulate a droplet of water hitting the surface of the water reservoir and causing perpetuation/ripples on the surface. I'm using the "Two phase flow-level set" physics module. I want to quantify the perturbation in a graphical way so that I can plot the averaged surface perturbation in the y-direction with time (it's a transient problem). But I don't know how to do that, I mean which quantity to plot against time? It should be a 'distance vs time' graph, but I don't know which property to plot. Do I have to define a custom variable of some sort? Like that in the boiling water example where they defined something called the "Interface Delta" function?

Please help.

https://dl.dropbox.com/u/6219893/demo_col.mph