As a denstist that is starting a practice, your business plan will typically reflect negative cash flow during the first few months as your practice revenues build.  At some point, your practice will begin to break even, and then experience positive cash flow.

To determine the amount of working capital you will need, use the forecast in your business plan and total the negative cash flows during the period during which expenses exceed revenues.

Working capital available should provide you with the cash flow necessary to run your practice until revenues begin to exceed expenses.