It is very important the archer be at the correct physical position before measuring the arrow's length. It is possible, based upon the archer's posture and drawing side back muscles to be at anchor but either too short, just right, or too long drawn. A new archer or one that is over bowed often is under drawn with the drawing arm elbow too much forward of a line from the bow grip, through the anchor and through the point of the elbow. Most archers overdraw with the elbow in the line or behind (towards the back) this line. I know some archery books show the proper position is with the elbow exactly in line but it should be 1/2-1" in front of this line to allow for back motion (tension is a bad word) to execute the shot so that anticipation and eventual target panic is prevented.