Out of the box this uses the level Android requires every phone mic to be tuned to, so it reads close without any meter. Free sanity check: a quiet room at night is about 30 to 35, normal talking at 1 metre about 60. For the last few dB: borrow any iPhone, install the free NIOSH SLM app, hold both phones side by side on a steady sound and press Match.
A phone mic has no absolute reference, so precision comes from calibration: put the phone next to a calibrated meter (or a 94 dB calibrator), then type its reading and press Match. The offset is saved on this phone. Echo cancellation, noise suppression and auto gain are switched off so the signal is raw.