Check to make sure your 1/4 inch foot is sewing an exact 1/4 inch seam first. Sew two pieces together and measure the unit to see if it measures the correct amount. If it measures correctly then the pattern should work out. If your unit measures a bit short, try moving the needle position over one spot on your machine (if it has this feature) and do another test. If you use Aurifil thread to piece, it is very thin and makes more accurate piecing also.
P.S. remember that if you move the needle position, you may need to change the foot and the needle plate (if using a single hole plate) so you don't break a needle hitting something.
Last edited by Tartan; 03-15-2013 at 07:13 AM.