It should measure 1 1/2.Because,the scant makes up for the space in the seam the fabrics take up.Another way to try it out,is,cut 2 squares the same size.Put right sides together.Sew E X A C T U A L L Y on the diagonal.Now cut 1/4in. beyond the stitchinq.And press the 1/2 square triangle open.See how your seam is thrown off by the thickness of the fabric.You need to sew just a scant to the right of the diagonal to get it exactly on the diagonal because of the thickness of the fabric.