This is an excellent question. I would recommend you start by researching how much traffic is available for your specific keyword strategy. Assuming that you invest in good SEO and can achieve good early search engine rankings this could give you a good starting point.
There are many websites that can give you daily traffic estimates for specific keywords on Google, Yahoo etc. Take the estimated traffic and apply the formula Tiana outlined for you and you have something to work with. Here is a traffic estimator on a site that I can recommend and a link:
---Tiana Nelson, MBA, CMA, CFM
Summit Executive Center