(edit: what @beerfaced just said) The trick with set focus is that you need to be sure it’s hitting your material surface. Look to see where the laser dot hits. It you’re trying to focus close to an edge, the dot may hit in the wrong place and measure something unexpected.
As for me, I tend to enter heights manually too. I use a step/depth measurement with my digital calipers, zeroed at 1.3", which is my tray height. No fuss no muss, and highly precise.
If you’re not familiar with how to do a step or depth measurement: