What is it that you meed to know, precisely?
Do you need to know exactly where in the centiles she lay? For what purpose? You know she’s below c.75% of her peers because she’s below 100, and she’s a distance off (as the peak of the distribution will fall above 100, the distribution graph will be rising above her scores and falling below them).
Are you worried about eg setting in her secondary? Do they set on the basis of SATs? As a non-grammar in a grammar county, the highest attainers will not be in her school, so she will be closer to ‘the middle’ of her school’s ability range than she would be in a full comprehensive in a non-selective county.
I have already posted about how her trajectory may well be seen by her secondary. One real advantage for her and for you is that she has a genuine opportunity to ‘beat her predictions’ and reap the self-esteem that comes from that.
Also, the narrow range of subjects vs secondary could hide her strengths - art, drama, sport, music, DT etc etc are all utterly unlinked to her SATs, as is ‘being a genuinely nice person who tries her best and everyone likes’.