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.
Focus the camera first, any way you like: set the lens to infinity and aim at something far away, or turn the ring by hand until the ground glass looks as sharp as it gets. What matters is that the image on the glass is at its sharpest when you start. Then hold the phone over the finder the way you normally do, keep it still, and the app will try every focus setting of the phone and keep the sharpest one for this camera. It takes about fifteen seconds.