As Lee mentioned, for 2 sprockets it is pretty easy to calculate center distance. For 3 or more on the same chain, it gets quite a bit more difficult.
Dr Joe created a useful excel spreadsheet for calculating this, check out his whitepaper:
http://www.chiefdelphi.com/media/papers/1598
Its not the most intuitive interface, but will let you find the appropriate spacing to avoid half-links, and get a tight chain. It also creates a plot to visualize the chain run to make sure it matches your design intent.