Flat-Earthers use “8 inches per mile squared” to calculate the hidden height of a distant object. It is not the proper formula as it ignores the observer’s height & refraction. It will result in an erroneously larger hidden height than the correct amount.
A 2-m-tall observer will be able to see a 5-m-high object at 10 km. However, the 8-inches rule erroneously tells it should be entirely hidden. It is the source of a lot of flat Earth misinformation.