Even if your business is really taking off - meaning that sales are growing at a phenomenal rate, your company could still (and usually do) face cash shortages as your bills (current expenses and the huge expense related to the growth) may be outpacing your cash inflow (actual cash into the business, not receivables - the actual amount in you checkbook)
Note: while managing costs is paramount to a successful business - so is managing revenue - ensuring that your receivables are closely matched to your payables.
So, the bottom line is simply this: You don't need an exact figure (precise amount of capital) before starting your business or specific costs amount (these will change daily). No matter what you estimate, if you are willing to make the hard choices and manage both revenues and expense, you can still grow and succeed. It is all up to you - not merely based on some start-up figure.