That phrase is about knowing the
maximum dosage beyond which the side-effects render the trip unpleasant; along with the
threshold dosage below which you don't get any effects.
Taking "too little" Ritalin recreationally — i.e. not meeting the threshold dose — doesn't result in ADHD-like symptoms.
Unless, y'know, you have ADHD. In which case it's not "recreational usage", you're just self-medicating.