I developed this tool for myself and it has turned out to be really effective (for me). I hope it is as helpful for others.
The idea is that to perfect your first layer and the first layer squish, you can use this tool to see how much squish you have and adjust it (adjust your z-offset). Furthermore, you can find this exact squish again in the future after you make setup changes.
The slots on the tool are all at 0.1mm increments, so as long as you have some squish to begin with you can see/measure your squish and then use that as a reference for printing. In fact I use a different amount of squish for different materials and I use this tool to make sure I am at the right level of squish from time to time and also whenever I change materials or setup.
Definitely watch the video as it is hard to explain but it works.