Niksammy,
My own personal thoughts.
I think pretty much everybody agrees that any new I-485 filed in August or September is not going to be approved this year and will not show in the Demand Data until adjudicated in several months time (in FY2014).
For me that is not the really the problem when thinking about when dates might retrogress in FY2014.
I think most people also agree that CO can not set a COD for September to use up the remaining visas that assumes all preadjudicated I-485 cases will be approved. Therefore there will be an element of contingency built into the date set.
That means that when the visas run out for this FY, there will almost certainly be people left behind who are Current and preadjudicated but for whom a visa was not available.
All those people will be counted as Demand going into FY2014.
Looking at past USCIS October Inventories, that number appears to have been about 2k, of which most should have already been in the Demand. I admit that is rather imprecise, but CO should have the means to make a reasonable estimate ahead of time.
Without releasing spillover, EB2-I quarterly allocation from October 1, 2013 is only 757 visas for each of the first three quarters.
For me at least, when retrogression starts and how much it will be initially, is driven more by the numbers left behind (or the estimate CO makes of that number) and the PD breakdown of those cases. New I-485 cases submitted late in FY2013 are a secondary consideration and those number will be added to the demand at a later date as they are adjudicated.
If all the queued up porting cases get approved in FY2013, than the retrogression might not initially be as severe as last year.
TBH, I can't make my mind up about what will actually happen.