I'm working on success data for a team of sales people, it's a fairly straightfoward calculation:

Column 1: Name of salesman
Column 2: Number of successful sales
Column 3: Number of unsuccessful sales
Column 4: Total number of sales calls

Success rate equals to the successes over the total (=column2/column4).

What we've noticed is that we have people who have made many sales calls in a month with just a few successes, they sport a low success rate. Another employee got a total of 2 sales calls, 1 was successful which gives 50% rate. In reality the first employee was more successful as he got more business in.

I'm looking for a formula that can combine the success rate with the volume of business every employee is attracting to the company. Any ideas?