12.92gb is what you really have, you can fill 12.92gb of your things but then you need the space for pictures and cwm backup, I always try to keep 1.5gb free for videos and pictures on the fly and then 900mb is for my cwm backup. 600mb for android/data and 400mb for images thumbnails which leaves me 9.5gb for my things.
Most of you have a cloud, I have a home server for music,images,videos etc, so I don't have any bandwidth for services once I leave my wifi and I need the battery to make it throughout the day.
Its enough if you manage your files keep what you need on there, 100hours of music is enough
Nexus 5, Nexus 7 2013