What do I do if the camera keeps capturing the baseboard on my wall?

The camera isn't a genius, but it does try its best to find where the edges of the paper are. It's trying to find lines that look like the paper's borders. Sometimes it might think that the baseboard is the line. To fix this, try tilting your smart phone slightly so that the main surface of it is parallel to the floor. Since each measurement happens within seconds, feel free to try different angles!