These lasers should focus to a point at a working distance of about 50mm. I think the easiest thing to do is fire up your 3d printed and create a simple ruler/standard that measures 50mm, maybe add some legs so it can stand on it’s own on your work surface. Then put this ruler on your work surface and bring the laser down until it just touches it. The end of your laser should have a knurled cap that holds the lens in place(spring loaded). You have a few turns of thread here to fine adjust the focus point at that work distance. That distance will always be fixed so you can just ‘zero’ your laser z height against your rule before you burn from then on.
Before you create anything , take a ruler and confirm that your focal distance is close to 50mm. It has to be pretty close to start with because there isn’t that much adjustment room on the lens assembly.