Just an FYI - JenniePenny wrote a very clear tutorial on assembling those corner units.
If you look at the last picture, you can see she has 1/4" seam allowance at the bottom point of the yellow square. When the corner unit is attached to the side of a square or to a HST, the point won't get chopped off.
http://www.quiltingboard.com/t-146734-1.htm