I want to engrave a piece of wood that is 0.806" think, but it is engraving out of focus. I’ve tried with different heights, and with and without the tray. I understand that WITH the tray it’s up to 0.500", which is the limit is giving me for the focus height.
Next I wanted to engrave the handle of a hammer but that would probably be a disaster as well.
What am I missing here?
You might want to check this link:
https://community.glowforge.com/t/tutorial-how-to-cut-without-the-crumb-tray-honeycomb/7528
There is another “no math” version too.
Either one should help you. Good luck 
Here is a calculator you can use to figure out how high you have to raise the part and what focus to put into the GFUI.
Quick cheat sheet: Subtract your thickness from 1.4", that will give you the minimum standoff you need. Then, add your standoff thickness and material thickness together, then subtract 1.4", that will give you your effective focus height.
T=Thickness
S = Standoff thickness
1.4" - T = Min S
T + S - 1.4" = Effective Focus (must be between 0 and 0.5")
The links above show you how to do it generally, but for this specific example, I’d pull the tray and then put in 3 pieces of 1/4" plywood stacked on top of each other and then put your piece of wood on top of those. you now have 1.556" of height. Subtract 1.4" (the height of the tray bed - at least on mine, you can measure yours but they all seem to be right about there).
Select Unknown Material and Use Uncertified Material and set the focus for 0.155" (I rounded down) and laser away.
Now if your 1/4" is actually thinner than .250 you’ll need to make an adjustment. For me, my 1/4" seems to come in at 6mm a lot so the overall height of three sheets is only .708" so my focus would end up being .114"