I'm a current Year 12, did 2U and 3U last year at a school somewhere probably around 70-90 in the maths table. Forgive me, it was so long ago and my memory is a little hazy, but as a rough guesstimate I would have had 110-117/120 for 2U and 77-81/84 for 3U. When I sat HSC, was 1st in 2U with average about 95% and 2nd in 3U with average about 88%. Final results were 97/100 in 2U and 49/50 in 3U.
If you're planning on a 97-98% result in your externals at a top 40 school, I assure you that you have nothing to worry about.