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.
While I still recommend the business planning process, I caution you to realize that a beautifully crafted document does not always equal business success. I've worked with many entrepreneurs who successfully launched without a plan, and some with beautifully written plans that never materialized. You and your business idea are unique. Your planning process will be unique as well. Be wary of one_size_fits_all advice or pronouncements from experts about how you should proceed.