SQL percentage calculation
I feel that rofl pretty well covered how this could be done differently, but I want to point out what I feel to be an issue.

You really shouldn't be formatting the decimal results as a string. Formatting should be done in whatever application consumes the data you're sending it. As a user, I would be pretty upset to receive a "percentage" that I couldn't do any mathematical analysis on without first splitting out the "%" character, dividing by 100, and then formatting it as a percentage natively. 

Select CONVERT(decimal(5,0), 1.0 * SUM([SalesCount]) / [Total Leads])