Yes, that is correct. That is how the HSC makes up 50% of your final mark. This is why during the year you should put in all the effort you can to get that ranking, because if you are ranked 7th in the class, then decide to work work work, and get the highest HSC mark - well, you aren't getting that mark, the person who got 1st in the class in school, gets that mark and you get an adjusted mark according to the graph made the BoS which graphs your school v HSC results and you get an adjusted mark according to your place in the line.