This data is specific to Thunder Laser Nova Series head assemblies but the principle applies to most chinese gantry CO2 lasers.

Here is more on how the ThunderCAM plays well with autofocus to maintain correct calibration:

NOTE: If you do not have or use this type of autofocus you can save a z position in lighburn and use material thickness to calculate your focal plane numerically.

On the mini 60 there is no good way to mitigate the focal point changing as the bed is fixed. the thickness of the material would affect the focal point.

One option would be to make your reference focus the thickest material you cut. let's say 1/4" or 6mm. set your camera to the surface of the 6mm. then, if you want to use the camera with another material, say 1/8" or 3mm, add 3mm spacers under the material to bring it up to the focal point. 

Another option is to save alignments for the thicknesses of material you use then load that camera profile for the specified thickness