I have one of these handy gadgets. The kit comes with a "probe" that looks like a piece of wire with a 90 degree bend at one end. You insert the probe in the holder so the tip of the probe touches the gauge tip. You then zero the dial. When you insert a piece of brass between the probe and gauge, the dial will measure the thickness anywhere from the web to the mouth.
I use mine in the "run out" mode. Despite your best efforts, every now and then you will get a cartridge with excessive run out. This is more common in factory ammo than reloaded ammo. Any cartridge with excessive run out is sure to be a flyer. A great tool if you are loading for precision.