It isn’t possible to work this out exactly BUT the best case scenario for a score of 90 would be that half the students who score less than 100 score less than 90 (ie the graph is flat between 80 and 100).
So the best case scenario is that at 89 she’s in the bottom 12%.
As it is very unlikely that the graph looks like this, and much more likely that the gradient of the graph gradually increases between 80 and 100, a reasonable guess might be that your daughter is in the bottom 10% of those who took the test for spelling and grammar.
Her position in the other two tests would be slightly higher, so maybe bottom 15% or so would be a sensible guess for you to work with.
If your DD’s scores are very much lower than the school would have expected, they may well attach a note to them as part of transition. However if she was anyway receiving intensive small group help, then these sound in line with expectations.
The question is what you do next. Reading is critical - is her low score likely to be pace? Vocabulary? Ability to infer? Keeping reading as a priority over the summer, perhaps through the summer library challenge, and also reading lots of non-fiction (reading short pieces at pace and making a game of ‘give me 3 new things you’ve found out’) will be helpful and will give her a head start against those children who do nothing between SATs and September.
Similarly, Y7 Maths content overlaps strongly with primary content but may be structured or presented slightly differently. Making sure that she remains fluent in times tables and number bonds, and can carry out the written methods for all 4 operations without thinking, will give her a good start.