Yes you have it just about right. It differs with the cohort sitting the test. If you get a very bright cohort it pushes the percentage needed for a particular school higher.
On the day there are so many factors which affect the kids and only you know how your DC is likely to cope under the pressure. I spent time confidence building before the day which I think was as valuable as doing the practice tests. Talking about what it will be like and simulating exam conditions at home.
It is not that helpful to worry to much about what is needed for where because it is all up in the air until you get the results. What I tried to do was to be flexible about which schools I would be happy for my son to go to. As long as he passed I would have been happy so he could go to any of the grammars. We were not aiming for Pates or STR, just to get it done and passed. The rest we would figure out after that. So much changes in the post results period regarding places, scores needed and appeals etc.
Just keep up the good work and build their confidence so they have the best chance of performing at their very best.