You could use Markdown before conversion to identify formatting/structuring. Heuristics can then be switched off. Calibre is fully Markdown aware. I realise that this doesn't answer your issue directly but if you use plain text sources a lot then you may find Markdown generally useful.
Full details can be found at
http://daringfireball.net/projects/markdown/
(You don't need to download Markdown, just understand the syntax - which is very simple.)
For a quick test:
Identify chapters in your book by starting each chapter line with the prefix "## " (hash-hash-space) then convert using:
Structure Detection
Insert page breaks before = //*[name()='h1' or name()='h2']
Table of Contents
Level 1 Toc = //h:h2
TXT Input
Paragraph style = "off" and Formatting style = "markdown".
(Textile formatting could also be used as calibre is also Textile aware. It's a bit more complicated than Markdown, but more flexible.)