



 
Hi Joe, I drew a diagram of your regular octagon. If $d$ is the distance from $A$ to $B,$ in your example 500 mm, and $s$ is the side length, the distance from $B$ to $C$ then in a response to a question from Lewis Penny showed that \[ d = \left( 1 + \sqrt{2} \right) s.\] Use her expression to find $s$ for your octagon. Triangle $ABC$ is a right triangle so you can use Pythagoras Theorem to find the distance from $C$ to $A.$ Write back if you need more assistance, Harley 



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