The measurement method in the manual is somewhat simpler (page 3-31, or near that). Measure from inside the first roller to inside the 11th using a vernier caliper. They take the arithmetic out of it for you by providing a specified limit of 6" for this measurement, which is 2.5% over length. (a new 520 chain would measure 5.85"). If you want to reduce your wear limit to a lower level, calculate it as a percentage of 5.85. Remember to measure it in multiple places and check tension consistency all the way around. The wear pattern shown on Ironman's web site IS in fact simple sprocket wear, and does not indicate a stretched chain. First, note that the sprocket rotates counter clockwise as shown. The wear is apparent from 7:00 to 9:00 o'clock in each tooth valley, on the drive bearing face. If the chain were too long, the points of the teeth would also be worn, and/or pulled to the left. As they are, the chain has never contacted them, because the pitch is still short enough to keep the roller from reaching the tooth point. In the manual, also on 3-31, is an illustration of a worn sprocket, slightly exaggerated for effect. The tooth at the left was worn by a chain still within limits, the tooth on the right was damaged by a stretched chain.