Use El Burns methods that use oversize then cut to size. You cant go wrong that way.
Two and a half inch half square triangles have so many design uses that they are a great way to learn.
Take 2 fabrics, one light one dark. Cut into 6 in squares and put them togather right sides facing.
Draw an X corner to corner then sew 1/4 inch on each side of line.
Cut in half at the 3 in measure on both sides of square +. Then cut on drawn lines. You will have 8 over size half sqs that you trim to 2 1/2 inch squares.
Put them togather however you want to make an 8 1/2 square. 8 inch
finished in quilt.
Be sure and watch lots of videos on
http://www.quilterstv.com