The problem with your rule of thumb is that there can be lots of external factors at play. Like, what if the person happens to be deeply gifted and/or intereste

The problem with your rule of thumb is that there can be lots of external factors at play. Like, what if the person happens to be deeply gifted and/or interested in some school subjects, allowing hyperfocus on them? What if there is major external pressure pushing them to take care of their home, do well at school etc? I think ADHD is a spectrum, which includes those with "ultra-light" symptoms as well. Whether those in the lower-end of the spectrum need stimulant medication is a different matter.