I currently use Ubuntu Linux on a standalone laptop - it has no Internet connection. I occasionally carry flash memory drives between this machine and the Macs that I use for network surfing and graphics but I trust my family jewels only to Linux.
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.