With the springs bearing directly on a beam axle, the spring rate is the wheel rate. Obviously, lateral spring spacing affects the roll stiffness.
With an IFS (or in the case when the spring is mounted on a trailing arm attached to a beam axle), the linkage ratio must be taken into account. If the linkage ratio is, say, 1.5 to 1, the wheel rate is the spring rate divided by 2.25 (or 1.5 squared).
Stopped to do a little googling and found this:
His "motion ratio" is my "linkage ratio." Also, rather than work with trigonometric functions, it's usually easier to measure the distance from the pivot to the spring centerline on a line perpendicular.