How do you use a spirit level?
Lay the phone flat with one long edge along the surface you are checking. Tap Read Both Vials, then wait a second for the reading to settle.
The long vial shows tilt across the phone. The short upright vial at the end shows tilt along it. When both bubbles sit in the middle, the surface is level in both directions.
The large number is whichever direction is further from level, because that is the one to correct first. Across and Along show each direction on its own.
Worst drop converts the larger reading into millimetres of fall over one metre. This is the figure to use when you cut a packer or a shim.
Why does a level need two vials?
One vial measures one direction only. A worktop can be level across its width and still tilt toward you along its depth.
A single reading will not show this. That is why a builder's level has a second vial set at ninety degrees to the first.
Place the phone on the surface rather than holding it. Even a steady hand adds about two tenths of a degree.
If you prefer to watch one bubble instead of two, the 2-Axis Level shows the same information in a single round vial.