givio I used the following link to figure the size of squares need for the corner triangles. I cut 2 -18" triangles then cut those once corner to corner but it did not give the the 36 1/2" I needed so I had to add a thin black border to get it to that measurement. So if you did not want to add a thin border you would have to cut bigger and then that would make the center block look like it was floating. I hope you understand what I am trying to say, sometimes I confuse myself. LOL So if you cut maybe 2 -20" squares it would get you to the 36.5".
http://quiltville.com/onpointmath.shtml