You could use the Square in a Square method if you don't think you could do it well with using oversized triangles and then cutting it down. The SiS, by Jodi Barrows, method really works well. I don't use it much because I think it wastes material. But, it is the easiest method for insuring a good 'fit.'
Here is a video of one way to make SiS:
http://www.hgtv.com/video/super-fast-rotary-cutting-video/index.html I love this technique for scrappy quilts.
This site has a chart for the different sizes:
http://quiltville.com/sqinsqtutorial.shtml
Another site:
http://www.frugalquilting.com/square_in_a_square_quilting_block.htm Use method #2 for your block it is much neater.
Also:
http://sleepyowlstudio.wordpress.com/2011/04/03/square-in-a-square-quilt-block-tutorial/ but this needs that specified size for the center square. Likewise:
http://www.askpennyhalgren.com/IKM/questions/135/How+do+you+make+the+Square+in+a+Square+Quilt+Block %3F
You might understand the math involved by looking at how this one does the different SiS sizes:
http://www.bellaonline.com/articles/art172248.asp
Yes, I know, I had too much time on my hands. Oh well.
Enjoy,
ali