I noticed that many brand new users don't take any time to explore Calibre (menus and option settings) before starting their Library with the throttle wide open and no course laid in. (Some of you choices will depend on the reding di=evices you wish to support)
My advice for all new programs, Calibre included: "Take time to look at all the menu choices, see what options settings are available, and always explore the Right click (context) menu choices.
Change (almost) nothing at this time. (That setting, may not really do what you think it does. Read the balloon help twice).
Using: Ubuntu(32 bit):Oneric,Precise and XPpro SP3, W7HP(64)- - Libre Office w/Writer2EPUB