Just a quick scribble as he puts it! But really its a fair amount of work and very clever. With Google and computers at our disposal its all too easy to take things like this for granted. Given the size of two sprockets and length of chain, work out the distance between the centres of the sprockets. Easy!
But this is proper old school engineering. No computers, no Google. Not even a calculator! You need to know the formula and do the maths.
Be like Bob!