There is a neat app for the iPhone called BalusterPro that makes
calculating baluster spacing a breeze. You just enter your distance
between newel posts and the width of one baluster. Then you drag a
slider to count your number of balusters, and as you do, it
simultaneously calculates your spacing. You just stop when you reach
your desired spacing. Then tap a button to show a list of all the
centers. I have been using it for a month and it is very cool. Just
go to the iTunes store and search BalusterPro.