Although this moment could be seen as manipulative, it's nothing plot breaking or a deterrent to the MC's general personality, as this could also as easily had been a quick overstep, seeing as he's a high schooler just trying to not get one of his 100 soulmates killed. It's generally better to...
I tired looking up "seven-five" tones: https://en.m.wikipedia.org/wiki/Seventh_chord
Assuming the source and what Rosa is analyzing are related, this would make her a prodigy on par or even greater than Mozart/Beathoven.
If true, what she refers to 7.5, is probably the tone of a Major seventh...