I came up with the expression “top 10% of your mind” many years ago, when I was teaching the What Color of Your Parachute workshop.
It is said that you only use 5% of your brain. But, of course, you use all of your brain most of the time, but not for something worth using a precision instrument even the stupidest person possesses.
The truth is that 99% of your usage, what you see, what you say about what you feel, what you think is not new, and not accurate and not worth thinking, because it leads to no benefit.
I have students that make this phenomenon very easy to observe.