What you need

Steps

On the Bench screen, choose the aircraft, then tap Incidence.
Lay the iPhone on the fuselage datum, such as the wing saddle, top toward the nose. Tap Capture.
Lay the iPhone front to back on the wing near the root, top toward the leading edge. Tap Capture.
Lay it front to back on the stab, top toward the leading edge. Tap Capture.
Optional: hold a straight edge along the motor shaft, lay the iPhone on it, and tap Capture.
Read the decalage under Results, then tap the Save button.

Keep the model still between readings. Moving it changes the datum.
On a curved wing, lay a straight edge across the leading and trailing edges and put the iPhone on that.
Flip calibration takes each reading twice, turning the iPhone around in between. It cancels tilt from a case.
Nose up is positive. A negative thrust line means down-thrust.

Incidence: The angle of the wing or stab compared with the fuselage datum.
Decalage: The difference between the wing and stab incidence angles. It affects how the model trims as its speed changes.
Datum: A flat reference on the fuselage, such as the wing saddle. Angles and distances are measured from it.
Thrust line: The direction the motor or engine shaft points. A few degrees of down-thrust or right-thrust is common.
Leading edge: The front edge of a wing, stab or control surface.
Chord: The width of a wing or control surface, from its leading edge to its trailing edge.