The didn't know about the clutch wear tool but the thinner gasket won't make the slave push too hard against the release fork.

Self adjusting slave cyl like these have a deep bore with a light spring pushing against the piston to keep it extended. The spring is light enough to keep the slack out of the system ( release bearing in constant contact with the pressure plate fingers ) and allow for the fingers / release fork to move towards the cyl as the disc wears. The difference in new to worn clutch disc release fork height is much more than the difference between the home made gasket and factory shim.

If the flywheel was machined / slightly different clutch used, the measuring tool will be invalid.

Think of this as a disc brake caliper against new pads / worn rotors and new pads against new rotors. There is enough travel in the calipers to take up the difference in rotor thickness.