Thought I had added these pictures but apparently not. Here is the refined design with a small pin added to the centre of the dowel. Its actually a 1.5mm drill bit. The 1m aluminium ruler has a matching hole so I can hang the ruler off it. The hole in the ruler is dead centre of its width.
The picture shows the top just because its easier to take the picture. I can use the top of the ruler but on the exhaust side there is not enough room to drop the ruler to use its top edge so I use the bottom edge on both sides of the bike.
The ruler measurement markings are irrelevant all you need is a metal strip and use a pen to make a mark on its edge. A strip of metal is better than my first design of using a tape measure.
In the first picture I have pulled the ruler away but when I measure I push it all the way to the spindle end. Interesting as I drew a mark around washer with the old chain and you can still see it and it shows the difference between the new and old chain stretch (wear
).