If you are going to cut you plain squares at 6inch they will finish in quilt at 5.5inch. With 11 blocks across and down, your inside quilt will measure 62.5x62.5 without borders. For your half square triangles, you would have to cut your squares at 6 3/8" then cut in half. This is according to my eq program. If you have the easy angle ruler, then you would just cut your squares for the half square triangles at 6inch. here is the site for the easy angle
http://www.simplicity.com/p-1467-eas...tool-6-12.aspx#