This page explains setting and corner triangles -
http://quiltville.com/onpointmath.shtml - remember to use your FINISHED block size when using the table. I'm not sure whether you mean that you cut your fabric into 7" squares (so 6.5" would be the finished size) or 7.5" squares (so 7" would be the finished size). In the example from the book, the finished block size was 9.5" and the squares were cut (unfinished) at 10".