I'm a math whiz, but not familiar with the 10min block to which you refer! :-o
Using 1/4" sa, adding a 1 1/2" strip to each side of a 5" square will give you a 7" square. Adding a 2 1/2" strip to each side of the same 5" square would give you a 9" square. (Add the widths of the two fabrics you are sewing together, then subtract 1/2" for the two 1/4" sa's. Sewing three fabrics together, you have two seams, so you subtract the 1/2" times two.)