One is that you have to take time, lots of time, to let an idea grow from within. The second is that when you sign on to something, there will be issues of trust, deep trust, the way the members of a string quartet have to trust one another.
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.