I just saw a spacing method on Ask This Old House that I wish I knew years ago. I can't think of how many times it could have come in handy.
If you get a chance to watch Episode #1009 of ATOH, watch the section part where Tom Silva uses an 2" wide strip of elastic fabric to layout the spindles for a porch railing he is building.
First, he marked the unstretched elastic strip every 2 inches. He then held one end at the beginning of the bottom rail and stretched it until he had a "just under 4 inch" spacing between the marks, making sure that a mark lined up at the other end of his bottom rail.
Since the elastic strip stretched uniformly, the spacing between the marks stayed consistent.
At that point he transferred the marks from the elastic strip to the bottom rail and screwed a spindle at each mark.
No complicated math based on how many spindles per length and how far from each end they have to be to have the same spacing throughout, etc.
Pretty neat trick.