Used Injector springs will exhibit a variation in spring tension to the point that you cannot count on a x sized shim to change the Pop/Opening Pressure the same on all of the Injectors.

You test an Injector; write down that Pop/Opening Pressure. You use the FSM recommendation to guess the size shim you might need. You put that Shim in and you test again. If the Pop/Opening Pressure is not correct you take the difference between the original Pop Pressure and the Pop Pressure with the new shim and see how much shim thickness it took to cause a change in Pop Pressure.

Now you have a little formula for your individual Injector and go off with your Micrometer or Vernier Caliper to find the next thickness of shim based on your formula.

And, you do that with each Injector. It is easier to do than to write about.

Added:
This is from the Factor Service Manual concerning changing the Spacer (Shim): "Increasing the preload by 0.05mm increases the Injection Pressure by about 3.0 bar positive"

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84 300D, 82 Volvo 244Gl Diesel