Essentially, the higher your ATAR aim, the less room there is for error. In your case, your goal of achieving a 99.80 ATAR means that you must ideally perform at the highest possible standard in your school-based assessment tasks/exams (allowing you to ensure sufficiently high ranks relative to your cohort) and in your HSC exams. The exact impact of your performance on your chances of achieving your ATAR goal depends on your ranks following your first set of year 12 exams and the weighting of each (where a higher weighting means a more significant effect and a lower weighting means a less significant effect). With that being said, many students who do not perform at the standard that they were hoping at the start of year 12 manage to make a comeback throughout the year and, in many cases, end up achieving their ATAR goals.
Based on this, you should be able to make a comeback. Since you attend a high-ranked school, the possibility of improving your rank, and the extent to which you may will depend on two factors, the first being the performance of your peers in their school-based assessment tasks/exams (which is beyond your control), and the second being your own performance in your school-based assessment tasks/exams, which you must emphasise as you progress through year 12.
The link that Unovan provided above is to carrotsss's ATAR calculator, which is a tool that considers your raw marks in your trial exams or past papers in providing ATAR estimates. This may be a useful tool for you in terms of determining the approximate raw marks that you need to achieve and which would be consistent with your ATAR goal.
I hope this helps!