The Android OS and preinstalled applications should take up about 4.6 GB of total space. I did a factory reset on my 16 GB unit to make sure. My box shows 12 GB free and it's a 16 GB NAND. NAND on this unit should be the same as the MXIII so I reckon a 8 GB NAND would put you on about 4 GB free more or less ... You can also use Apps2SD in Google play to move apps and games to SD. Or remove preinstalled apps on your own. Who needs Skype, Facebook and the like on Android ... My opinion anyway

It can also come down to how its partitioned, I remember earlier Android boxes had issues where 1 GB was available but this was due to the fact that the NAND acted as both memory and storage. My old Jynxbox was setup this way ... Installed too many apps and the box suffered performance wise.
Hope this helps.