What's the appropriate way to calculate conversion rates from traffic to leads?

A very simple calcuation: # of leads/amount of traffic. If you have 5 leads out of 100 visits to your site, your conversion rate is 5%. 

You might also want to track a second stat of conversion of leads to sales. This will give you an accurate way to describe your sales funnel: If it takes 100 visits to get 5 leads, and it takes 5 leads to get 1 sale, you will need (theoretically) 50 leads to get 10 sales, and 1000 visitors to get 50 leads. This kind of calculation can help you forecast your leads and sales based on traffic.

A note: these kind of forecasting systems are only a guideline, don't take them as fact. You should recalculate your conversion rates on a regular basis (quarterly, monthly, depending on the rate of sales) to get a more reliable picture of your funnel.