Here is five advices how to get better measurements:

1. Hold the camera forward when you walk away from the object, don´t point it downwards. The phone tracks the position of structures with the camera.

2. Measure in daylight conditions. The camera tracks the position of structures and it is easier during daylight. New phones have better cameras and with them you could do measurements even before dawn and after dusk.

3. You should at least walk away half the height of the object. If you think the object is 100 feet high, walk at least 50 feets from the tree.

4. Aim at the base of the object. If you could not see the base you could aim below the red marker.

5. The app works on Iphone 6S or newer. You will get more accurate result with a newer device. 

You could take a look at the Video Tutorial Below.