Based on the 2011 statistics, you needed an aggregate of 456/500 to get an ATAR of 99.5.
This aggregate is the sum of your best 10 units AFTER scaling. Since the top-band results that are published in the newspapers are not based on these scaled marks, you cannot use them to judge the level of scaling in subject for the purpose of calculating an ATAR (in fact no students sees their scaled marks).
From the 2011 stats, the PERCENTILES that you needed in each subject to score a scaled mark of 45.6/50 in 2011 were as follows:
General Maths - No-one
100 IPT
100 SDD
100 English Standard
100 Music 1
99 Ancient History
99 Biology
99 Visual Arts
99 Society & Culture
99 Studies of Religion 1 unit
99 Business Studies
99 Legal Studies
99 Engineering Studies
99 Geography
99 PD/H/PE
98 Chemistry
98 Modern History
98 History Extension
98 Drama
97 Economics
97 2 unit Maths
96 Adv English
96 Physics
95 Ext 1 English
92 Ext 2 English
92 Music 2
83 Ext 1 Maths
67 Ext 2 Maths
Of course, the higher the percentile required, the more difficult it is to score that result. As these numbers are rounded, the subjects stating the 100th percentile had a cutoff for 45.6 somewhere in the top 0.5% of the candidature. In Ext 2 Maths, you needed only score amongst the top one-third of the state to score 45.6 (and hence be on target for 456 and thus an ATAR of 99.5)
(BTW, I hope you realise how few people score 99.95 - only about 50 in the state.)
This aggregate is the sum of your best 10 units AFTER scaling. Since the top-band results that are published in the newspapers are not based on these scaled marks, you cannot use them to judge the level of scaling in subject for the purpose of calculating an ATAR (in fact no students sees their scaled marks).
From the 2011 stats, the PERCENTILES that you needed in each subject to score a scaled mark of 45.6/50 in 2011 were as follows:
General Maths - No-one
100 IPT
100 SDD
100 English Standard
100 Music 1
99 Ancient History
99 Biology
99 Visual Arts
99 Society & Culture
99 Studies of Religion 1 unit
99 Business Studies
99 Legal Studies
99 Engineering Studies
99 Geography
99 PD/H/PE
98 Chemistry
98 Modern History
98 History Extension
98 Drama
97 Economics
97 2 unit Maths
96 Adv English
96 Physics
95 Ext 1 English
92 Ext 2 English
92 Music 2
83 Ext 1 Maths
67 Ext 2 Maths
Of course, the higher the percentile required, the more difficult it is to score that result. As these numbers are rounded, the subjects stating the 100th percentile had a cutoff for 45.6 somewhere in the top 0.5% of the candidature. In Ext 2 Maths, you needed only score amongst the top one-third of the state to score 45.6 (and hence be on target for 456 and thus an ATAR of 99.5)
(BTW, I hope you realise how few people score 99.95 - only about 50 in the state.)