Hi Pardha,
This isn't a formula question, really - it's a "how well do you know your customizations" question.
There isn't a real formula. There is a broad, and untested formula - that I attempt to use, though. I call it 75% of what was originally necessary for the Modification.
Now - take that number into consideration, and exception, for each modification and the span of time necessary till go-live. If your modification took ten weeks, initially - and you want to go live in five weeks - you may want to target at least 150% of your initial resources.
The rule of 75% is my personal experience, not based on anything scientific - maybe it's more-based on intuition (and, recall - I didn't think it would snow in Dalton,Georgia, this year).
How many organizations 'really' know how long the original 'enhancement' took to place into Production? I didn't think so....
(db)