I believe the peilican tool has a light spring inside it.
the tool is a known height , it is used in place of a valve spring... the dimension in the "window" is added to the known height. Then shim calcs are done
I have used something like this for years, its a valve height micrometer. Replace a valve spring with this, screw it out until the valve is seated, and light resistance is felt, then directly read the measurement. Then calculate shim thickness requred.
