Here's another patch, inspired by popular demand.
This patch allows to change the margins around the book content. For the simplest use case, you can now influence what the Kindle understands as "fewest/fewer/default" words per line. You can also hide the progress bar, if you are so inclined. Finally, this patch also affects the PDF display, so in principle, PDFs can be made to display on the full screen.
I attached a few "before/after" screenshots (see below). The result is largely depending on your individual settings, though.
INSTALLATION: copy the attached file "ixtab_margins-5.1.0.zip" directly to your Kindle's opt/jbpatch folder (do NOT unzip the file!), and add the corresponding entry to CONFIG.TXT.
CONFIGURATION:
With the default configuration, this patch does not change anything at all. To change the behavior, you have to modify the file "ixtab-patch-margins.txt"
inside the zipped file. After editing, make sure that you re-zip the patch correctly, then restart your Kindle.
SCREENSHOTS
Before:
After:
The PDF files are mostly for verification: The central box should always be square, and the borders are a visual indicator of the actual document dimensions.
Update: I attached the actual pdfs, and the code to produce them - maybe they are useful.