What is the proper procedure, step by step, to determining if a valve shim needs to be replaced on 1989 420SEL?

I have the tool for checking the height of the HLC. Engine is cold, valve covers are removed. Cam shaft is rotated so the lobe is at the top. When I put the tool on the HLC, it shows the shim needs to be increased in size. Then I removed the rocker, did not change the shim, replaced the rocker and checked the height again. The HLC rose and now the tool shows the shim is the correct size. It seems to me, the tool is checking the height of the HLC, but it keeps moving depending on if the rocker is installed and how long the rocker is installed.

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1989 420 SEL

Before you start measuring with lifter height tool...

You need to rotate the engine so the lifter in question is completely compressed for
at least 30 seconds. In other words allow the cam lobe to completely open that valve.

That will bleed out excessive oil that might allow the lifter to hold open a valve,

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