How full before adding?

[pmac3072]

Hello,
How full should I allow my N5200BR Pro to get before adding an additional drive? I currently have two 500Gig drives in a raid 1 setup with about 70Gigs left. I have a drive on the way, but I am stationed in Iraq, so it could be weeks before I get it. I only ask because I have more data I need to add, but would like to ensure I keep what I have and not have it refuse to go to a raid 5 setup when the third drive arrives...
Thanks,
Paul

[pmac3072]

Does it matter at all? Can I get it completely full, Maybe 4 or 5 Gigs left, then migrate a new disk in? Will it hurt it or just take more time?
Thanks,
Paul