The quilt I am working on right now calls for this square in a square and I have been struggling with this same thing. On the website discussed in this thread i found a video on making this square in a square. You might want to check it out cause it will take the worry out of us having the triangles the correct size. I am going to make my squares the way this video says to do it from here out. Also their chart will help us to know what size squares to start out with. I am so glad you wrote in about this cause it solves a big problem for me. The quilt I am working on now is my first with the square in a square, flying geese and hst. I have done the geese before and really messed up that quilt with my geese that were all out of wack. I since bought rulers to make my geese and now they are perfect. Now if i can conquer the sq in a sq and the hst I will be good to go. I am having a little prob with my hst. Here is the link for the video.
http://quiltville.com/sqinsqtutorial.shtml