I get what you say. But this is not the way SE sites do it.
A bit of history
"Off topic" used to be just that, off topic, and was presumed to cover anything out of the scope described on the Help page. Later, it turned out that sites usually had a few points which people frequently expected to be on topic, but were not on topic. The system was then changed to cover the most usual ways of misunderstanding the site's scope as three "specific off topic reasons" per site. Note that a site can never have more than three of them. The advantage of doing so: it leaves an officially-looking canned text which educates everybody that "this common topic I associate with the site's topic is not welcome here".
How is it handled now
It is still expected that any question which does not fall in the scope of the site as described in its scope-defining help page should be closed. If it falls under one of the three most common reasons (from which we only have one defined until now), that reason should be chosen. But if it does not fit one of the three (and that will probably cover most of the off topic questions), you just choose the option to manually write why it is off topic instead of using one of the three (or less) pre-written reasons.
"off-topic as per Help page" is not a specific closure reason and should not be defined as such. It is presumed that every single question closed as off topic fulfills this condition. A specific close reason should specify a subtype of question which is off-topic per the health page, ideally one which is both 1) commonly asked, and 2) not immediately obviously off-topic.
On an unrelated note, I think that this specific question about the gel dosage is on topic. But this deserves its own Meta question.