Using calibre editor (right click > Edit book), insert an xhtml file before each part you want to split with the following text:
Code:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
</head>
<body>
<h1>My Book Title - Part 1</h1>
</body>
</html>
Replace the part in
blue with the appropriate text.
Note: You can insert a new empty xhtml by going to File > New File.
Now using the editor's TOC tool (Tools > Table of contents > Edit table of contents), create headers from the parts you just inserted, after which you can use the plugin to split at desired points.