Getting 141 in a paper doesn't mean that the child got 100% in the exam, though. They may have done, but unless the raw scores (pre standardisation) are known (which they are not) you cannot hazard a guess.
The top scoring child might get 80/80 in the VR exam. That would be 100% and as top scorer they would get 141. However, the top scoring child might only get 60/80, being 75% and yet they would still get 141 as their standardised score. To add a degree of complexity, you might get two kids each getting 80/80 on the VR paper yet, if one is an August baby, that is the child that will get 141 and the other one, being a September baby, might only get 136, say.
It totally depends on the cohort each year and, as the schools don't divulge the raw scores, a guess at the percentage required is impossible to make.
I think if your child is getting 80% or above on the NFER/Letts practice papers then you are in with a good shot. However, I can base this only on having 2 daughters who have passed, rather than on vast statistical evidence!