GoDaddy isn't the best place to register a backorder. They only capture a domain if it drops and stays dropped. I would suggest using Pheenix.com, unless the domain has backorders on either SnapNames/NameJet. If they have backorders, you should back order the domain there also. By "dropping from registration" I presume you mean actually dropping and not just expiring. Expired domains drop 81 days after expiry. At least for .com domains.