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