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
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,
Pretty neat trick.