^ Don't get me wrong - I'm not saying "trust only your eyes," by any means.
What I mean is that one should't take a leveling bubble - or any such device - simply "at face value/at it's word," *EITHER*.
What I'm trying to say is that one should always use their own good sense when performing the critical tasks of leveling/placing their detector(s)/jammer(s), and while good tools/instruments can and will help significantly towards optimizing one's setup, they should not be the only items used to make such determinations, either.
One should assess that their vehicle is parked on decently level ground, that their vehicle does not present special considerations for rake/stance, as well as both eye-level and true-level their detector in-vehicle. It should't just be one or the other.