Next, build in the cost of doing business. Determine your capital equipment costs by adding up the total cost of furniture, coping machines, computers, answering machines, fax machines, modems, etc. Include a generous allowance for software. If you you're not sure about software, price a business software suite, a good drawing program, and a statistical package of choice and multiply by 3. You will be surprised about how much software you will buy and also how productive it will make you.
Most people skimp here and that is a bad idea.
When you have added all of the items in this list, divide by three. This will give you a rough estimate of the annual capital costs of running a business. Note, many items will have a life longer than three years. On the other hand, you will be surprised by how quickly you will need more equipment, faster equipment, better equipment. Identify the variable costs.