Chris,
These gauges work in two different ways. I used to use a Gates gauge, but it was a total pain in the ASS. I now use a Continental gauge, this works in a different way and is much easier to set up. Tensioning belts is now a doddle. I could try to explain each technique, but it would be easier to visit the relevant websites of these manufactures. Alternatively try to get each one demonstrated at your garage...

Good luck Paul...