This works for both fork and shock springs, any spring as long as it’s steel, not Ti. By the way, there is no such thing as a progressive spring. The way they achieve this effect is to pack some of the coils closer together, so they coil bind early, leaving the remaining coils to offer the higher rate.
If you don't know the rating on a spring, you can use the following
Formula:
((diameter of the wire)^4) * 1470000
---------------------------------------------------- = spring rate
((diameter of the spring)^3) * (number of coils)
^4 means to the 4th power
^3 means to the 3rd power
diameter of the spring or coil is measured center to center of the wire, measure the OD of the coil and subtract the wire diameter. All dimensions in inches, returns rate in pounds per inch. This formula works because all of the various spring steels used in suspension springs have the same nominal bulk modulus. This will not work for hi-tech titanium springs. you'll notice that with the number of coils in the denominator, this implies that two springs identical except for number of coils, the one with the most coils is softer.
