I can't figure the math on my binding.... I thought to end up w/ a 1/2" binding- you start w/ a 2 1/2" strip-- fold in half- press- line up the edge w/ the edge of the sandwhich and sew it on 1/4 ". When I practiced this-- I end up w/ 1/4" on the front and bigger on the back. What am I doing wrong????