Spectator, Thanks for the detailed post.
Quick questions:
When you/Q mean 12K ROW backlog? you mean PERM backlog? because, if it's 485 then the Visa would already be accounted for and we don't need to double count.
My take:
Assuming that PERM applications from 04/01/2010 in ROW are the only ones in ROW that need Visa allocation:
There are ~17,500 ROW applications: of which if
70% are EB2: Then this will result in
13125 EB2 PERM.
Resulting number of 485 apps: 13125 * 2.2 =
28875
I see that there'd be a min of 6 k Fall-across here? ( I thought mine was very conservative

)
PS: IMO, any ROW PERM certified before 03/31/2010 & not filed 140+485 is automatically expired. Since ROW was always current last year, Visa number is allocated as soon as the 485 app comes through.