Sorry knew someone would say that, I'm confused too!
As in most people you tend to go grew on sides first don't you, on her sides and mostly through the back she is very light grey, if I was to match it up to a colour chart she is as light, and has the tone of a 10/8. Through some of her parting area I would describe it as a slate grey along the lines of a level 8.
What she wants to archive is to bring the parting area up to match her sides being the 10/8. Because her hair has been previously bleached I felt if I was to put 10/8 on her middle length and ends it wouldn't hold, that's why I was asking about using the 9/8.