Yes but a guess. The spreadsheet takes into account the current has rate, which includes some asics already, and was guessing about 20 Ths more by the time BFL ships. I'll make excel files available soon. I have a better calculation that I plan to post soon.
IC, I missed that tidbit. 20Th may be rather low given that Avalon's batch 2 40Th/s is scheduled to begin shipping sometime between tomorrow and May 5 and ASICMiner is expected to add 5 - 13Th to the network around the same time. I believe what BFL actually has on hand is one wafer of chips only, though perhaps they've made some progress in that area as I do not follow their unofficial releases.