some good observations about dart spine being reported here. It seems to me that the methods we use to approximate spine can only be rough indicators of dart dynamics in flight, because the launching of a dart involves a series of complex forces.
In a basic sort of way, I guess one could make a dart stiffer by shortening the small end and adding to the point end, at least if the new center section was stiffer than the old. An alternative might be to just stiffen the center third of the dart, as is, or of course just start with bigger shoots and work them down till they fly right. seem like my quiver eventually contains some softer darts that get used for easy target throwing, and some heavier darts for distance and wooly mammoth hunting