I would like to answer that, but since my German isn't all that good and Dräger writes most of their stuff in German, I haven't been able to find out. Their recommendation is very short: “Push the OK button and do a passive test.”
Probably because I have more technical data from them, I was able to discern the other manufacturer's recommendations. Their recommendation was two inches for the best result. They're not suggesting you can't do it further back, but if you want the best result, you install the collection cup on the top and you stick it within two inches of the individual's mouth.