Well, all levels are off to some degree. The easiest way to center it is to flip it around 180 degrees. If the bubble is off the same in both cases, it is level. This is an OLD carpenters trick. No level is ever bad, just the user doesn't know how to use it...ha ha.
The water level works, along with a trench filled with water.
A string, taught between two points will tell you if it is off, too. Simply put a couple drops of water on it at center(fold it in half to get center.) They will migrate toward the low end. I think the Egyptions used a plumb bob and a "T" as opposed to a bubble. 'Corse, this dates back a few thousand years, but still functions just as well. Lots of other ways, but, I really think this is trying too hard. (You do remember how to bisect a line with a compass??)
If it is that close to level, don't camp there. Chances are, in a rain you will get wet.
My thoughts only . . .