It needs to be tweaked for any level it's applied to in order to adjust for scale, fog, and desired level of detail. This version was adapted to VCTF-Sandstorm, one of the maps that comes with UDK.
I used the shader provided in this post as the basis for mine, and this post helped as well. This is another example I found more recently.
Click for full image
It's also possible to use the alpha component of the scene texture node instead of the scene depth node (the scaling values must be changed too).