I can't really give you an estimate as i'm inexperienced but
1.You can go on Topscores.info and search your school and the number of distinguished achievers in a given subject each year.
2.Here you can see how close you are to the band 6 range when accounting for your rank and the number of band 6s
3. Make a reasonable judgement on your marks and then enter it into an atar calculator
hope this helps