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.
Cheers