I don't think anyone would know with any degree of certainty...

I don't think anyone would know with any degree of certainty because the answer keeps moving due to the variable IO price over LOM (Life of Mine)
FMG would maintain financial models using reserve tonnes, grade, mining costs, forecast iron ore prices over LOM and price discount for each of their mining areas. These models would be rolled up for an aggregate FMG LOM. The main assumption to get right is the cutoff grade for each mine to be economically viable. Future IO prices will determine the LOM and will the most difficult input to forecast. So whilst running a highly efficient IO operation is important it's the IO price which will determine the future financial success of the IO division. Being a price taker their future is mainly beyond their control. Product differentiation e.g. green steel may help.