Practically all decisions in business have a numerical element to them. The most obvious example is the ubiquitous Business Plan; but far too many people think that the important bit is the words at the front rather than the numbers at the back. But make no mistake, anyone who is going to back a business turns straight to the numbers section to see where and how much money is going to be made.
Unless you are a genius, it is pretty unlikely that you will get the numbers right first time. After all you will have to consider all sorts of contingencies; for example variations in rent, salaries and cost prices. You may have different sales projections depending on your price point and you may have to allow for different interest rates or fees depending on the amount of money you borrow or commit to the business. If you are trading globally you may have to consider fluctuations in exchange rate and factor in different transportation costs.