Hello, I could use some help understanding focal length and adjustments to my file in order to engrave into the bottom of a wood tray.  

To understand my object, I have an inexpensive basswood tray.  4mm material thickness, 110mm x 160mm x 43mm.

When I measure the distance and use the inner tray where I will be engraving, the laser extends and knocks into the item.  When I use the highest dimension of the tray, the focal length is wrong and there are no markings on my object.

Is there a way to manually compensate for these conflicting settings or is this just a limitation of the tool?