With any good deck you are going to want to properly space your ballusters. After much searching I have found two links which greatly helped me in the process.
This one allows you to put in all of your specifications and gives you the center line of each picket to mark on the railing as well as the # of pickets.
Because the above referenced site gives you the remainder inches in a decimal you can use this conversion calculator to obtain the correct fractional inch. It even tells you how to do it without the calculator, if you didn't already know. Of course we're talking about fractional inches so a good guess would work too.
Hope this helps!