Darren - Clay % for ponds

We've moved onto a new homestead of about 15 acres and I'm looking to do some earthworks of various kinds and some ponds. I haven't done any soil testing and am not sure of the clay content of our soils yet. But, in your experience what percentage of clay in soil is needed to form a good seal on the bottom of a pond?

And, to put a plug in and maybe win the chapter 2 ... for those who are not aware, the documentary that Darren and his wife and daughter put together is now available on disk or video on demand at http://www.polyfaces.com/. You can view the trailer there, too.

30 percent is the very minimum