That gives us 16 possible focus heights : 0.000, 0.028, 0.056, 0.083, 0.111, 0.139, 0.167, 0.195, 0.222, 0.250, 0.278, 0.306, 0.334, 0.361, 0.389, 0.417.
Per Glowforge, those focus capabilities for the unit are sufficient to accomplish its cutting function with materials <= 0.250".
So, essentially you can be within +/- 0.014" of your desired focus, and it should still cut without issue.
A very in-depth discussion can be found here:
The “Material Thickness” setting, however is different. While that is used to set the “default” focus (which is then rounded to the nearer of the 16 possible values), it is most importantly used to de-warp the bed image for better optical alignment.