It's interesting formula but there's a lot of variables which are actually unknown and tough to predict. Calculation is good and give some insight but at the end you have to take the risk.It depends on how much you're looking to earn! Try this formula:Exact Local Monthly Searches X expected click through rate onto your site in SERPs X expected conversion / adsense clicks rate X amount you'll earn per conversion / clickThen think what it'll cost you to promote it, and the amount of your time it'll take up.