if it’s fuzzy then yeah probably the focus is wrong. If you can post a pic of the engrave that went wrong and an image of the recipe file we can probably narrow down the cause.
As for how high to raise it, you need it to be in that sweet spot of -.0- 0.5" above the crumb tray height. For most people that’s right around 1.8-2.3 inches from the base of the Glowforge. There’s a simple way to measure this, provided by the ever-clever @dwardio:
After ensuring your board is in that height range, I’d recommend using the auto-focus feature in the UI and you should be good to go on height/focus issues. Alignment left/right and up/down is a separate topic, which is a little beyond what you were asking.