“Forecasting business revenue and expenses during the startup stage is really more art than science. Many entrepreneurs complain that building forecasts with any degree of accuracy takes a lot of time–time that could be spent selling rather than planning. But few investors will put money in your business if you’re unable to provide a set of thoughtful forecasts. More important, proper financial forecasts will help you develop operational and staffing plans that will help make your business a success.”
This article from entrepreneur.com explains how to build financial forecasts when you’re just getting your business off the ground and don’t have the luxury of experience.