In this step we measure the opening of the gauge to make sure it maintains the 1:1.6 ratio. And we verify the gauge against known golden section architectures.
As you can see in the picture below, I've opened the gauge to a somewhat arbitrary width of 2" and measure the width on the other side of the gauge. The other side is about 3.2", which is what we expect (2" * 1.6 = 3.2").
Additionally, from the picture of the Parthenon in Greece, it is clear that the gauge accurately measures two aspects that follow the golden section.