Aim for something you can realistically achieve.
In relation to ATAR, if you've been get E's and F's throughout school you shouldn't be aiming for 99.95 going into year 12. Similarly if you've been getting A's you shouldn't be aiming for a 60.
Most people know what ATAR bracket they will fit in (given ~5-7 or so leeway below/above), you should aim for the upper range of this bracket.
If you set out aims which aren't achievable, you'll be mid-way through the year, knowing you can no longer get it - reassessing what you can get - this would be quite demotivating for most people.
With this being said, if you set them low, you won't be motivated, you won't work as hard as you can and won't get results that truly indicate your true potential.