only shim the MAIN SPRING!!!
...BTW your Bieber avatar is awesome.-Malone
From what I gather the main spring compresses a little on low RPM, but mainly it is the higher RPMs that it affects. I never had a tach, but I am guessing about 2,500 the main spring starts to limit fuel and will continue until 4,800-5,300.
enough boost is when you have 3 dimple marks in the hood from the valve cover nuts..
As I read it the Bosch book says above idle the intermediate spring is the one in play.It's really a beautiful system.