Arun,
It is good thinking and essentially the sort of approach I used when I first started analyzing the figures.
It assumes that the USCIS Inventory is the entire universe of applications, which of course it isn't, and that all applications held by USCIS are in the Inventory.
As an example, Consular Processed applications need to be added to the figures (at different % for EB2 & EB3).
Additionally, the last USCIS Inventory does not include any PWMB, who still have to file an I-485 or CP application. For Jan 2007, that figure isn't very high, but it becomes a factor nearer the end of the current backlog.
Essentially, what I do is very similar when I convert PERM to I-485/CP numbers, but uses a few more assumptions. I've tried to lay them out in this post
http://www.qesehmk.org/forums/showth...oved-in-FY2011
Welcome to the forum by the way.