I personally guestimate the marks that a student attending a school of 'x' rank would be getting. For example a student with school rank 500 with subject rank 4/20 would be presumed to be at around a high 80s range, wheras a person with school rank in top 10 and subject rank 4/20 would most definitely be assumed to be achieving 95+. You can also play around with atar calculators and check how cohorts perform in each subject. This is basically how i do it, a guesstimate approach but it usally falls within a few marks of peoples ATARs so at least it works. If you want other people's opinions i suggest you make a thread asking how do you calculate atar based on ranks.