Trust your gut instinct over spreadsheets.
There are too many variables in the real world that you simply can't put into a spreadsheet. Spreadsheets spit out results from your inexact assumptions and give you a false sense of security. In most cases, your heart and gut are still your best guide.
Microsoft has had two goals in the last 10 years.
One was to copy the Mac, and the other was to copy Lotus' success in the spreadsheet - basically, the applications business. And over the course of the last 10 years, Microsoft accomplished both of those goals. And now they are completely lost.