When I first started quilting, I was told to add 7/8" to the square, draw a diagonal line, then sew 1/4" on either side. However, depending on the position of the moon and the stars (just kidding). Now I add 1" to the finished size, so if you are making a 6 1/2" HST, I would cut 7" square, sew on either side of a drawn line across, cut, then I would trip to 6 1/2", it's an extra step, however, the accuracy of my piecing has improved with this method. The same is true of my blocks, I make sure I square them up to the unfinished size. I hope this helps and I hope I haven't confused you.