OEMs and suspension designers care about suspension leverage ratio (mechanical advantage) AND spring rate, which when combined give the wheel rate. Tuners who don't change the actual suspension geometry only care about spring rates as that is all thay can be changed.
Depending on suspension type, geometry and spring placement the leverage ratio can be (almost)constant, linear or non linear. Typically you can only get a rough estimate of this by a quick visual inspection and quick measurement of two distances. However, given the M3's nearly 50-50 weight balance roughly equal wheel rates doesn't surprise me.