Originally Posted by olis

I tried it and it worked beautifully. Try this:

1) Go to Home on your Kindle (it is important that you have all your books closed)
2) Attach the Kindle to your pc, find the file \system\com.amazon.ebook.booklet.reader\reader.pre f and change the margins as you please.
3) Unplug the Kindle, go to settings and make a restart (do not open any book before the restart).

Now you should have the margin width you chose; changing font size, font typeface and line spacing doesn't reset the margins value. On the other hand if you change "words per line" it does reset the margin to the default value (40pixels).
Basically this has solved the problem for me.
Thank you all guys!!