If an online business, you need to go into detail how you will attract customers to your website. General statements like "I will use Face Book ads and email marketing" will contribute almost nothing to helping your cause unless you have detailed statistical analysis of tests you have conducted or of another similar business you have been associated with. If you do not have any data upon which you reference your estimates, it could show lack of proper thought to the remainder of your business plan.
You may not have thought much about your competition or outsourcing some of your work, but things like that will impact your ability to make a profit. And you will find this especially so in the beginning phases of your business. Even you are just opening a lemonade stand in the front yard, you will still need to know what Susie is selling her lemonade for on the next street over!
The expected revenues and expenses for at least a year should be projected in the cash flow section of the Financial Plan. It's better to make conservative predictions rather than be too optimistic when it comes to cash flows. As part of this section, a break_even analysis is essential. This is the "amount of units sold or sales dollars necessary to recover all expenses associated with generating these sales." (NxLevel for Entrepreneurs, 2005) The formula for calculating the break_even quantity is Total Fixed Costs/(Price _ Average Variable Costs).
Do not try to write your business plan in one sitting. As I mentioned in the beginning, for a traditional brick and mortar business, it could take in excess of 100 hours to compile all of the information needed into a comprehensive but yet understandable document. For online businesses, probably not that long. But your final product should be well thought out, well documented and easily understandable.