Go back to my previous post:
http://www.quiltingboard.com/t-101870-1.htm#2589620
That is how the measurements are calculated.
To get a sense of the concept, try cutting a scrap rectangle, and a scrap square (3"), and follow those directions for cutting the QST's, and see how it all goes together.
It's a really weird concept and takes some getting used to. But once understood, it really can make your quilting life easier.