Storj displays the space as TB (same as drive manufacturers) while the Windows displays it as TiB, despite the unit of size is displayed as TB.
12.7 TiB is roughly 14TB, so I would say you can go ahead and set the node to 13TB for now and change the settings slightly again once the node will be full and drive wont.
You can also get rid of the data from decommed satellites as in some cases not all of it was deleted as mentioned here: How To Forget Untrusted Satellites - Node Operators / FAQ - Storj Community Forum (official)