71 Dart get 408/ Trick Flow Heads....

One thing i like....the spec sheet said they springs were installed at 1.90.....and my spring checker said 1.89,,,,,,add a .060 and a .030 shim bingo...I am right at the 1.80 where i want to be.......

Why shim the springs down? To get more pressure or get closer to coil bind maybe?