I would suggest further improvements for this useful plugin.
1) in the admin pane, a checkbox or something else ("enable auto-pagebreak", or similar), and a formbox ("subdivide pages each X lines"); I guess that's intuitive.
So, if one wishes to automatically subdivide an article in several chunks, without inserting the pagebreak snippet each time in the place where it will be divided (the "classic system"), one haves only to check that option, and the action will be done automatically.
In the case that this code's integration may involve any problems with the core of the cms, let's do that the user must enter the pagebreak command only at beginning, or anywhere else, of the article, and the script will be invoked selectively for that article.
I think that this would be a better option, 'cause, if one day one deselects the checkbox, the automatical subdivision will vanish from the articles wherein it was included.
Second consideration: in the case one haves already several articles subdivided with the classic system, along with the enabling checkbox would be good to put also an "override current subdivisions" checkbox... ;-)
Obviously, the automated system puts some problems into what is the system of naming for each chunk (you knowk, the "chapters", as we named then): I think that this may be surpassed with making that the writer will put at the very beginning of each article a sequence of this kind:
<%1-name of the chapter|2-name of the chapter|and-so-on%>
this won't affect the article validity in the case that the auto system will be disabled; simply, that tag won't appear.
Obviously, this method will override the basilar system (which won't have any chapter naming; so, that would be good in the case one wishes only simply a subdivision of an article in smaller parts for easy reading), in the measure that the chunking will be done according to the combo "X lines"+"number of chapters".
If the number of chapters are less than the default number of lines, the latter will be subject to the former, as far as the article will be anyway cut in parts of X lines each, starting from the beginning, but if there will be some lines in exhuberation towards the number of chapters, then the last chapter will include the remainder lines along with the ones of the default number assigned to each chapter.
In the case that the number of chapters were more than the default chunking declararation, then the latter will be adjusted in a measure to accomodate a subdivision subordinated to the former.
2) I think that nucleus' owners should considere to embed definitively a button in the edit panel, which invokes the pagebreak command (mean, like there it is in wordpress, for example); besides being boring to write it down manually each time, I think that this command it's a must have for any serious editor.