It's a comparator, so measurements are relative, such as when comparing fired cases to resized ones. In that situation, the before and after numbers on the caliper are just subtracted to learn their differences, which is how far the shoulder has been extruded down toward the head.
Other kinds of spacers can be used, but that flanged bearing journal is easy to keep in place with your fingers. Ideally, the hole through the bushing would be identical to the shoulder datum diameter in the SAAMI drawing, but because you are comparing rather than making absolute measurements, it doesn't have to be that exact. If you want to know how it compares to true minimum headspace, put a headspace GO gauge in.
Other kinds of spacers can be used, but that flanged bearing journal is easy to keep in place with your fingers. Ideally, the hole through the bushing would be identical to the shoulder datum diameter in the SAAMI drawing, but because you are comparing rather than making absolute measurements, it doesn't have to be that exact. If you want to know how it compares to true minimum headspace, put a headspace GO gauge in.