Here's the issue: How do you know anything is level? really... You could be in the water, the "level" will, not may, but will change depending on which side of the boat you sit on.
The only way I know how to do it is to use a level. And even then it might not "look" right. IIRC, those compasses allow you to wiggle them around once you put them on.
The compass liquid should also give you a built in "level."
Good luck, don't overthink this.
