STAT scores are not converted to ATAR/UAI scores simply because you are going into a different group of applicants, so it is irrelevant. As you is not competing against people going to Uni from High School.
You can’t give people who are doing an aptitude test (STAT) the same score/result system as people doing 2 years of study in different subjects (HSC).
STAT is for people to use as another entry, so you go into a specific pool. The majority of Unis will accept you on your STAT score. Not convert it to an ATAR.
The whole point of the ATAR/UAI is to rank/score people who complete Year 12, so STAT can’t even fit into it.
Unis in WA do a conversion method for the STAT to ATAR, as some Unis there DO convert it.
Here is their STAT score converter (so you can have a play and get an idea to put your mind at ease):
TISC Online STAT CONVERTER.
But again, nearly all Unis will look at your STAT Score, not convert it, not look at the percentile, as you are a completely different type of applicant. Even contact the STAT test people (ACER) and they will tell you that the STAT cannot be correctly converted as you are trying to convert two completely different systems into the same result.
I got into Uni on my STAT and it wasn’t converted to an ATAR, when I called Unis some people at the Unis thought it gets converted, but when you talk to someone else who knows what they are doing then you usually find out it doesn’t get converted. Majority of Unis (if not all) will not convert your STAT.