If you develop a good Profit & Loss Statement for your existing business for the current year or for the first year of your proposed business, you can use this data to actually project how much sales you would need to yield the income you want and the profit margin you want. You can plan ahead as far as you want. Sound impossible? It's actually pretty simple and can be pretty accurate plan.
In many cases business plans are very important but so much of the time it's a plan to try to convince someone else that you know what you are doing with your business like banks, investors, partners, etc. Now it's true that a well written business plan can also be a major benefit to your success as well if done right. It can guide you and keep you on track and can be the vehicle to get you were you want to be especially with so many outside forces now days that bombard you. A plan can be extremely important to your success especially when you look at the statistics that says 51% of small businesses fail sometime during their first 5 years.