Your cost of doing business is the money you must spend each month (or each year) whether you have any photo jobs or not. Thus, it represents the smallest amount you need to earn each month, on average, from the jobs you get. Our calculator lets you play with the numbers in various ways. In particular, it shows you what you must average on each job, just to scrape by and stay in business.
The hard part is determining what numbers to enter for each category. In some cases, we have suggestions in the popup Help screens (the icon). Otherwise, just make your best guess — sorry, establish a center-line estimate — and periodically update it as you gather more information and experience.