The hardest thing about an ATAR calculator is you need to get the study scores correct, and you won't know those until you get your results.
There are 2 main things you can do in this scenario:
1. Distract yourself from thinking about it
2. Plan your "what if" scenario for a low ATAR -> there are always multiple ways to get to a course career etc. Look up pathways and you may see it's not so bad
The ATAR can be scary but what's scarier is all of the meaning we attach to it, often meanings that don't need to be there.
The ATAR is a key that unlocks uni options. It does not tell you about the value of anything, intelligence, predict life success etc.
If you get a low ATAR, find a door that gets you on the path you want without needing that key.
Easier to say "dw" than to feel it but I hope this helps