I did say about because I cannot remember how many there is.

When I did Injectors in a Shop where I worked we had a box of about 100 unsorted different sized shim.
As the other member indicated you do a little math and you get a general idea of what thickness of shims your individual Injector needs.
You select the combination of Shims that aproximates the thickness you want.
You install those Shims and Pop Test again. If that does not work you now have a better Idea what will. You know that the last stack of shims changed the opening pressure X amount.
So you know what the Pop Pressure was when you started and what that
Shim pack measured.
And, you know how much change the new Shim Pack made in the opening pressure.
From that you can figure out what the next dimensions of the Shim Pack sould be.
If that works you start over again with Injector.

It is a Pain to do even if you have a bunch of Shims to work with.
And, yes there is always one that is not going to want to be balanced like the rest because no matter what Shim you use it will be off.
This is one of the reasons you are allowed such a wide spread in the balancing that is allowed.

I for got to mention I also had access to some used Injector Parts so I could als measure and swap those parts. Sometimes changing them would do the trick.

When you do this sort of thing on the Job you get used to doing it and it is not such a Chore.
Setting the clearences in a Detroit Diesel Blower is done with Shims (I did not like doing this) also. And every time you change the Shims you have to Pull the 2 Gears off very close to at the same time; and they need to go back on close to the same time also or they bind up and you can bend the Shafts.
You also need to do a similar type of Math to determine what Shims you might need.

When I worked at the one Shop I only did one Blower every 2 Months or so. When I got to My other Job I would be given 20 or more Blowers to rebuild.
I guess My point is that I also spent a lot of Brain Power thinking and anguish over some way to make it easier or faster.
What I really needed to do to make My job easier was to simply follow the procedure and change My attitude to accept it.
Once I accepted things the anguish went away.

__________________
84 300D, 82 Volvo 244Gl Diesel