Recently I have been interacting with small to medium-size business owners to discuss the topic of business opportunities whether it is an expansion of their current business or an evaluation of parallel income opportunities (like diversification). Our discussion usually starts with the business objectives and their assessment of the opportunity, I noticed that all of them, without fail, are using back of the envelope calculations, and nothing more, to evaluate their opportunity. No a single cash flow spreadsheet was used, the due diligence was a personal visit with the other company (to meet the owners, partners, etc).
This way of doing business clashes to some degree to what you see in business schools - where there is no investment without a lot of due diligence. We can learn from these successful people and use our gut to make business decisions. In our big corporations by having some degree of freedom in what bets we allow our trusted employees to place.
Like in everything, there is a threshold to the amount of money you can trust to your gut. If the amount is large enough, use the tools that MBAs, M&A people, investment bankers, VC have always used to evaluate business decisions. The amount should depend in your company's cash flow and in its risk culture.