Phone sensors can read slightly off out of the box. Calibrate on a surface you know is level (flip test: take a reading, turn the phone end for end on the same spot and read again; if the two angles differ, the phone needs calibrating). Phone cases with camera bumps rock; lay the phone on a straight board. Not a replacement for a quality level on finish work. Estimates only. Check measurements, product labels and local building codes before you buy or build.