That looks like a good tool SLO. I was using something like that back in the 80's while the rig was still on the frame rack before the "real" alignment. Caster measurement is computed by camber change during that 20 degree swing.
If your caster was zero, you would not have any camber change. A good way to visualize caster is to take a welding rod and bend it into a 90 degree angle say 4 inches from the end and hold the long end vertical (zero caster) and make it turn. The spindle stays level. Now tip it backward and visualize the short end being the spindle and turn it. Now you will see it move in an arc. (camber change)