Certain sites such as Wattpad encourage their writers to post their stories in numerous part that take between nine and twelve minutes to read. While it is possible to use the Readability Report to get a reading length estimate it is a matter of trial and error to find this exact length. Perhaps the tool could highlight an area at the beginning of the manuscript in between parameters set by the user. The author then marks where he wants the split. Then the tool moves on and highlights the next part of the text within the specified reading time.
This shouldn’t be too hard to integrate into the tool. I was a programmer for eight years and I had to do things like this on a regular basis.