To be honest, it really depends on the school and your cohort.
A decent (but often not very accurate) method of knowing what ranks are needed to get a 98+ ATAR is knowing how many people got a Band 6 in your subjects last year.
For example, if 10 people got a Band 6 for Chemistry last year, then you'll want to aim for around top 10 in terms of rank.
Given that your subjects scale relatively high (I did the exact same subjects as you), getting a Band 6 in all your subjects should land you a 98+ ATAR, provided that there are a few high Band 6s in your results.
For me personally, I had to aim for around top 5 for all my subjects since my cohort is quite small and we aren't a selective school, although we are in the top 100.