From my personal perspective, the study score doesn't matter as much as an ability to actually teach and differentiate content to the student's ability and requirements. You can get a 50 and be absolutely atrocious in teaching the actual content / being able to explain things in a way where your student will understand.
Personal examples:
* I got a 39 in IT Apps, went on to do an IT degree and now I teach it.
* I got a mediocre ss in Further Maths, went on to do stats and maths as a minor at uni, I now teach maths and i'm good at it.
I feel that to be a tutor - you must have an exceptional understanding of the study design or curriculum continuum (depending on VCE or <y10). You must know the areas in your subject where common mistakes happen and be able to explain them clearly to your student so they can avoid them. It's really about convincing your potential students that you are a person who can help them succeed.
No doubt a higher SS means that they know the ins and outs of the subject, but it shouldn't be the only measure.
So the answer to your question is... there's really no "minimum" study score a tutor should have. They should be able to demonstrate they've done well in the subject ("well" doesn't just mean a decent study score - it means content knowledge, it means understanding of the SD etc), and also have an ability to engage/explain in a way that is custom to the student. I don't agree with the use of the "40+" benchmark.
.. And really a tutor should be asking themselves this: Am I just doing it for the cash or am I doing it because I genuinely want to help others succeed like I have? If it's the former, please don't tutor and leave it to somebody who will put the care and effort in. Tutoring isn't a cash grab. Please don't misinterpret what I am saying here - it's fine to financially support yourself but if $$$ is the sole motivator, don't do it.