Students typically estimate their ATARs by considering their ranks relative to their cohort in each of their subjects. Essentially, the higher your rank in a particular subject, the higher your Assessment Mark (internal mark) is likely to be, which in turn makes a more significant contribution to your ATAR. Additional information, such as your school rank and the number of band 6's achieved at your school in each of your subjects is also relevant. This information helps in determining the academic capability of your cohort, based on which your ranks can be assessed. For example, a student aiming to achieve a mark in the band 6 range in a particular subject at a school where approximately 20 band 6's are achieved each year should be aiming to place within the top 20 students of their cohort. From that point, the higher their rank, the more likely they are to achieve a mark in the band 6 range, and the higher the mark value is likely to be.
The number of band 6's achieved at your school in each of your subjects can be publicly found on HSCninja, although students aiming for lower ATARs may be more interested in the number of band 5's (or lower) achieved at their school in each of their subjects. Unfortunately, this information is not available publicly, meaning that you would need to enquire about it yourself.
You can also use your trial exam marks to calculate your ATAR (see
carrotsss's ATAR calculator). However, I think that it currently does not cover all your subjects, so you may not be able to use it until (or if) the tool is updated to accommodate all your subjects.
In your particular case (and considering your subjects), maximising your chances of achieving an ATAR around 70 will likely require marks in the low to mid band 5 range. Based on this, you will need to prepare for your HSC exams to the best of your ability. The fact that you were able to achieve a band 5 in Music 1 is good as it constitutes a step towards achieving your ATAR goal.
You can determine your school rank by searching your school name, followed by "bettereducation".
I hope this helps!