Ian MacMillan says it is better to be "roughly right" than to be "precisely wrong."  That is a driving notion behind the BareBones NPV tool that MacMillan, professor of innovation and entrepreneurship at Wharton, and Rita McGrath, who teaches at Columbia University Business School.  NPV stands for Net Present Value, and the tool is designed to give innovators and investors a better sense of whether a business idea will generate necessary revenue, and for how long it will remain profitable in a highly competitive business climate.  MacMillan explains the tool in this video from Ideas@Wharton:

Rita McGrath has more information available on her blog, and provides a downloadable version of the tool here.