Another great resource, esshup! Yes I have Mike Otto's excellent book.

I religiously followed Mike's advice about testing the soil by making clay balls and clay pencils. My test hole digging went very slowly because I got out of the seat so many times to evaluate samples!

From Just Add Water:

"As a general rule, a site should have 60% clay, if you can possibly get it. This is ideal for a great dam and reservoir. Unfortunately, I very seldom have the pleasure of building a lake with all good material. Often there are lots of different kinds of soil on a single piece of property. Most engineers recommend a 30 percent clay content for compaction."

That is his advice for optimal sealing. However, he also notes that if your ground has big cracks during the summer months, then you know you have clay in your soil.

I am trying to figure out the correct clay% for a pond that will seal (against only 6' of maximum hydrostatic pressure), but won't crack during a dry out period.

Right now, I am considering going towards the lower end on clay content (10-30%?), and then if the forage pond is too leaky, it should be very economical to treat with the addition of some Soilfloc.

I did have several soil tests where I could make good clay balls that held together but contained some fine to medium sand grains. Some of that material would also roll out to "crumbly" pencils. I did have some material that rolled out to some pretty good pencils, yet I could feel that there was still some sand content in the samples.

I am pretty sure I have the optimal material on our property to make raised forage ponds. (Or I could make it by mixing two soils).

I just lack the requisite knowledge. (Which is not unusual for me!)