I agree that a minimum of 90 is good enough mark wise to show you know most of your stuff, provided you didn't fluke your way to that 90 lol...
I also do not believe in the "the higher the mark, the better the tutor" myth. The difference between say 90 and 95 (particularly in maths) can easily be primarily due to too many silly mistakes rather than a big difference in knowledge. It is also certainly possible for students to achieve better marks than what their tutor once achieved.
Also, the HSC mark represents how much you knew in Year 12 and is certainly not indicative of what you may know since then. I for one, know a lot more about maths today than what I did in Year 12, after experiencing the course with my students and also studying maths at university.
The key is how well you can actually explain something and how well you can communicate ideas to a student, something which is almost independent of the HSC mark. Unfortunately, you can't really prove your abilities in that in an advertisement, particularly if you're starting out, so it will be your HSC marks that will be judged by its face value hence it may be difficult to attract students at first if they're not too great.
However, if you can get good student results and prove your communication skills on them, you will definitely notice a higher interest in your services if you continue to tutor.
That's pretty much how most businesses start out...