Focus height is the difference between the top of the crumb tray and the tip of the laser - so .5 is max, because that’s physically the amount of space available
If you search the forums you’ll see a number of posts on how to cut without the crumb tray - and if you’re cutting something ~1.38" tall then your focus height will be 0 because that’s approximately where the top of the crumb tray is. If what you’re cutting is 2" you have to do the math to figure out 2 minus 1.38 = focus height
*each crumb tray has some variation in height - so measure yours to do this correctly
Check out: Problem for setting material thickness (without tray) for a visual aid 