Shimming the springs makes them shorter, while adding additional seat pressure. Think of the top of the spring as a fixed point (though that can be moved too with +.050" keepers, etc). When you shim the bottom of the spring, it just adds any seat pressure that's needed and shortens the installed height of the spring. The added pressure can be easily calculated based on the thickness of the shim used and the spring rate. You just have to be careful to check that the installed height of the shimmed spring still allows for enough lift and clearance needed before it reaches coil bind. I hope this makes sense.