Depending on where you are, if you have a clear sky I would use a ND16 or maybe even 32( if you can find one for the Phantom ). I use a 67mm variable ND Filter 2-32 for my Nikon 4K, it has a 60x Zoom so I am hoping to get some awesome video of it from the ground providing there are no clouds.
Check this video out from 35,000 ft.
http://content.jwplatform.com/previews/tiRaKPgQ-LGVHcJDb