Jim - Arron's sticky talks about 5%+, My mix has so much it has a peachy tint to it. So you don't need to get to worried if you put a bit too much in your recipe. One 8 oz cup to 10#s of dirt is 5%
You can contact nilocg , Colin is a great guy. He offers small portions of Clay, Potash and Dolomite. You can buy just what you need a forum member that can be trusted.