Australia builds a lead but when is enough, enough?

Australia are now building a lead, in a fashion we are more accustomed to. Ponting and Kattich have combined and after lunch on Day 4 have extended their lead to in excess of 250.

Now, in the past, this was the comfort zone. Now it's different though. In days gone by, when the Australian batting failed, I used to find myself thinking, "well we'll see how bad this really is, when the other mob tries to bat", and invariably, it turned out that Australia were doing pretty well, and would struggle.

Now this is all backwards. Australia are 1 for 130. What's going on? I'm sitting here thinking, if we can make this many, how many runs will South Africa get? We're going to need to be 500 in front aren't we?

The commentators on the ABC are saying just over 300 is enough - somehow I doubt it!