Ok imagine over distance is ∞ and under distance is 0.5m, where I put the two arrows, turn your focus so the two arrows are equal distance from the focus line, the small numbers below, 8 in this case, are the minimum dof needed to encompass both over and under, but I would recommend at least f11.
Here is your picture edited, I have moved the focus ring to where you would need it to be with the above numbers.
Aaah, sweet! that explains it! Sounds good. Is the distance "below" equal to distance above?
I've read something about an "virtual image" that you focus on so the distance below is not the same as above.
The search term you're looking for is Zone Focusing, or Scale Focusing (https://en.wikipedia.org/wiki/Scale_focus, http://digital-photography-school.com/ultimate-guide-to-zone-focusing-for-candid-street-photography). I've become better at it since getting my 120 folder that I have to set the focus manually on the lens before using the viewfinder to approximately frame. Unfortunately most lenses these days don't have any kind of decent focusing scale, although with a fisheye like the one you have unless you're just a few feet from the lens you can pretty easily get acceptable focus to infinity stopped down a fair amount.