Kat:
This is how you guesstimate your UAI:
1.) Find out from your Principal/Careers Advisor what percentage of an average year group from your school gets over certain benchmarks. Say, 1/3 over 90, another 1/3 over 80..whatever.
2.) Divide your rank in each subject by the number of candidates in the cohort. This will give you a decimal. Add the decimal from each subject and then divide by the number of subjects to get your mean percentile.
3.) Compare this figure to the data you got from your Careers Advisor. Say if your mean figure was 0.32, and 1/3 of your cohort got over 90 each year, then you could roughly estimate yourself as at about 90.