productivity – Measuring user’s focus or concentration

I have ADHD and something I’ve been wanting to develop is some sort of an app for myself that helps me keep track of my focus, nudges me as a form of stimulus, etc. The problem I’m not sure how to approach is measuring my concentration.

My first thought was using gaze detection/eye tracking but after fiddling with some of the repos I found on github, none of them were really accurate in detecting how focused I was.

Does anyone have any idea how this could be achieved? I considered something along the lines of putting a prompt on screen and looking at reaction time in order to measure focus, but I feel like that would just be an unnecessary interruption.