The Guy said it all

Forecast from the bottom up. Most entrepreneurs do a top-down forecast: There are 150 million cars in America. It sure seems reasonable that we can get a mere 1 percent of car owners to install our satellite radio systems. That’s 1.5 million systems in the first year. The bottom-up forecast goes like this: We can open up ten installation facilities in the first year. On an average day, each can install ten systems. So our first year sales will be 10 facilities x 10 systems x 240 days = 24,000 satellite radio systems. That’s a long way from the conservative 1.5 million systems in the top-down approach. Guess which number is more likely to happen.

http://blogs.openforum.com/2008/11/25/the-art-of-bootstrapping/