A focus score is easy to fake. You could tie it to typing speed, or mouse movement, or just make it drift up when the app is open. We did not want that. We wanted the number to come from your brain.
So we use an established EEG metric. It is called the engagement index, from Pope and colleagues in 1995. The math is simple. You take beta power (13 to 30 Hz) and divide it by alpha (8 to 12 Hz) plus theta (4 to 8 Hz).
In plain terms: beta goes up when you are alert and working. Alpha and theta go up when you drift or relax. So the ratio rises when you lock in, and falls when you fade.
The raw number has no natural top or bottom. So we map it to 0 to 100 against your own baseline. We freeze that baseline after the first 20 seconds. That means 50 is your normal, not some global average. Your 70 and someone else's 70 are not the same thing, and they are not meant to be.
One more note. It is a within-session number. It tells you how you are doing today, compared to you. That is the honest version, and it is the one we ship.