Honestly though, for preliminary, try your best but don't worry too much if you don't do as well as you expected. But be worried if you're on the middle-bottom of the cohort if you're in a crappy school.
I for one would think that if you did jack all for maths during year 7-10, you should be working a lot more in year 11 to fix up all the foundations (i.e. your algebra skills, as well as the year 11 2U course in general since most of it repeats year 9 and 10.). Can't say too much for the other subjects except that you just need to get used to the feel of prelims so that you can feel mentally capable of doing the HSC.
In my case, for English Advanced - beginning prelim rank = 150/170, end ranking was like 75/150, but external mark was 89 (1 off 90, fml).
Legal studies - 28/40 was my beginning of prelim rank, in the end I ended up coming 10/35 and got a HSC mark of 89.
For maths though, I was bottom half for the end of year 10 maths, because I never studied or revised for exams properly. Come down to year 12 and I come first in MX1 in the cohort. In MX2 I came 3rd, but meh.
Things can really change from year 10 to year 11 and to year 12. So make the most out of it.