The answer is built into the input
Suppose you guess five high HSC marks. The calculator returns a high ATAR. Lower the guessed marks and the ATAR falls. The arithmetic may be correct, but the output mostly restates the assumptions you entered.
Scaling is not the same as forecasting
UAC explains that historical scaling can estimate an ATAR for a set of HSC marks. That is useful for scenario testing. It does not establish whether those HSC marks are likely for an individual student before the exams.
A better question to ask
Start with what is currently observed: school, rank and cohort size. Estimate a range of course marks from historical school evidence, then convert each scenario through historical scaling. This does not remove uncertainty, but it makes the assumptions visible instead of hiding them inside guessed marks.
Official sources
NESA: How school assessment marks are moderated
Turn your ranks into a planning range
Compare your school, course ranks and cohort sizes with five historical scenarios. The result is a beta estimate, not an official ATAR calculation.
Forecast my ATAR