I think I 'heard' Mary Ellen Hopkins advice in Jan's discussion. She was trying to help quilter's understand that we can be successful when there were some small variations in our techniques for achieving the elusive 1/4 inch. There are so many things that cause the variation-thread count of the fabric, weight of the thread you use to piece, fiber content of the thread you use, the direction you press, the machine foot you use, the markings on the machine etc. etc. One of the constants I feel is important is to use a metal tape measure on a flat surface to check accuracy. I have a Craftsman roll up tape I have used for 25 years. The key is to check frequently and learn how to deal with your own small variances to get the near perfect product.