Taking the question as asked, rather than the general version of it. The dose-response is real but shallow at the top. Across STEP 1 and STEP 4 the gap between 1.7mg and 2.4mg is a couple of percentage points of body weight on average, and the average is carrying a wide spread — plenty of people at 1.7mg sit above the 2.4mg mean. If a dose is working and tolerable, "working" is the relevant variable, not "maximal".
If somebody has the primary source to hand I would rather cite it than paraphrase it.