You can't compare the BOS trials with the HSC. It's just at a completely different level since it's specifically aimed to challenge the students who have mastered virtually every single topics, i.e people who have the potential to easily achieve 95+ HSC. Even if you did do badly, it wouldn't reflect your performance with your HSC exams except maybe in a couple of Q15-16 type questions.
I recall doing it back in 2013 in exam conditions at Parramatta. I wasn't one of those state rank guys so I got rekttttt lol.
Got around 40% but then I got 94 in HSC (though I did make a bit too many silly mistakes but that's just how it is)
Just take this as a fun challenge, rather than a way to boost your ego or a way to measure your competence.