Hi there! :-)
Regarding your goal of Harvard Business School - as it's postgraduate, your ATAR and high school results are more or less irrelevant to whether or not you get admitted. If say, you want to do undergrad at Harvard FIRST, then a high ATAR would potentially work in your favour, but it still wouldn't guarantee entrance. You would need to score high on the SAT (which you can sit in Australia), and ALSO have a highly impressive resume of extracurricular activities / volunteer work / awards / leadership etc.
I'm currently in the US, about to commence my studies (albeit not postgrad, at Harvard, or in the business/economics field) - and from my experience with the whole admission process US colleges pay MUCH more attention to non-academic aspects.
I'd recommend you checking out Crimson Education which is very experienced in helping Australian and NZ students get into top US/UK universities.
As for how to get a 99.95 ATAR... the one advice I'd give is to pay attention to ALL your subjects, most of all English (or whatever english subject/s you're doing). As it is the only subject that's guaranteed to be in your top 4, it's the one subject you cannot afford to mess up (as I learnt the hard way). :-)