I don't believe SgtJohn claims that the 0.5 value for the Australian is entirely accurate. Rather the value is abstract, approximate - meant to give a rough idea.
It is easy to dismiss this on the grounds that the number allotted is not objectively representative - we know that it is not. However I believe that this is a useful framework that aids in thinking about these events.
In language we constantly use abstraction - for example, I may say that the human body consists of the head, the torso, the arms and the legs. Someone may interrupt me and say that I am wrong for ignoring other parts of the body, such as the eyes, nose, feet and internal organs. But sometimes we cannot list everything and need to rely on a more simple model to grasp complexity in some basic way.
Similarly here, the Australian is in no objective way worth half of Wimbledon. But it is hard to argue that it is worth as much either, and it is also not a good idea to ignore it altogether. In order to avoid unnecessarily complex numbers (0.75 and other varieties) it is probably the best idea to simply allot '0.5', without any pretention of total accuracy.