I posted my math on this on my blog. The post is a little long but it shows my method for figuring it out. Of course (since I ordered a BFL 50ghs) the whole thing assumes that the butterfly lab units will eventually ship. I think they will. If they don't the hash rate won't change much. But if they do ship it changes a LOT.
http://mineshaft.me/2013/04/initial-roi-estimates/Interesting read. One thing that bothered me while reading through the data presented that perhaps you could clarify. Was network hashrate added by vendors that do currently have a working product(Avalon & ASICMiner) taken into account?