I just made a quilt that requires strata (strip) sets. The sets were to finish at 10.5". Since they measured out at 10.25 I knew I had to move my needle one "tick" to the right in order to make my seam allowances slightly smaller. Another thing is to be extremely careful with pressing. Sometimes you can get little pleats and these must be coaxed open. Mary Ellen's Best Press will help here. Read Harriet Hargrave's new series, especially the first one, Quilter's Academy Vol.1 Freshman Year. I took a 2-day course with Harriet using this book and it improved my accuracy quite a bit. Harriet stresses that it does't really matter whether your seam allowance is an exact 1/4". What DOES matter is that the block finishes at the correct size. Also, I swear by Superior's Bottom Line thread. I use it top and bobbin. The finer the thread, the less room is lost in the seam. (Picture a rope vs. a narrow ribbon being folded into a seam allowance and you'll get the idea.) Hope this helps.