I set it up so that the string comes in contact with the corner of my mouth, when using proper shooting form. I incorporate back tension into form because it is the most accurate. When I have the string at the corner of my mouth I know that the string is the right length because I can use me thunb and index finger webbing right behind my jaw. The first referance point I always use is the position of my shouldr blade in referance to my spine. It is all defined clearly in "Core Archery" BYLarry Wise.