Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
"So if a boot drive needs 15%~ free how much space (% wise) does a non-boot drive need that is only used for large video files?"

This is only my opinion, but if the drive is NOT used for booting or the storage of cache or temp files (which have size requirements that can change as the apps that control them are in use), you can essentially "fill up" a "non-boot" drive until there simply isn't any room left to which to copy any more files.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.