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
Calculators use published historical scaling data to approximate an ATAR for a set of HSC marks. UAC uses raw HSC marks, not the aligned marks on a results notice, so published tables are not an exact conversion. Scenario testing also cannot establish whether the entered marks are likely for an individual student.
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.
Sources and editorial notes
Founder of ATAR Forecast. These guides explain school ranks, HSC results and the assumptions behind the forecasting tool, with links to NESA and UAC sources. These guides explain the process; NESA and UAC remain the official authorities.
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An experimental estimate, not an official ATAR calculation.