One trick we used to use pretty frequently was to assemble the suspension without the spring installed and measure the distance between the upper and lower spring mount pads at the ride height we wanted, and again at the anticipated full compression distance and/or the bump stop. That information, plus the weight carried by the wheel gives a spring vendor most of the specifications needed to recommend a specific spring.
One of my competitors used an oddball-looking device for measuring wheel loading that used a bathroom scale. The linkage reduced the load by a 4:1 factor, so a 300-lb. capacity scale didn't get overloaded. It was made of square steel tubing and aluminum plate for the wheel to bear down on.
Jerry
