This whole discussion about using cutting mats to measure probably explains why I never participate in BOMs. It has been my experience that if I recieve blocks from other quilters there are always several that are not the same size. It usually happens with pieced ones. It makes assembling them into a quilt, if even possible, very frustrating. My husband is a machinist who works with metal everyday. He says in order to be truly accurate with it you have to control things like temperture as metal contracts and expands. We all know that fabric stretches etc. After more than 40 years in quilting I know why I prefer wholecloth quilting above all others.