Place the phone over the finder, tap the spot you want to measure, then turn the focus ring. The green dot lights up at best focus.
Do this once for each camera. Set the lens to infinity and point the camera at something far away, so the ground glass shows the sharpest image this camera can give. Hold the phone over the finder the way you normally do and keep it still: the app tries every focus setting of the phone and keeps the sharpest one. It takes about fifteen seconds.