Height Obscured by Horizon
Computes how much of a distant object would be hidden by the curvature of the horizon on Earth. Also gives viewer distance to the horizon and other related information. Refraction is % of Earth curvature and is multiplied into the Earth radius until I can figure out how to get an appropriately curved Tangent line but the end result is the same. Common values used in surveying are between 7% and 14%.
Adapted from Mick West's 'Obscuration by the horizon' - ALL credit to him, I just tweaked things to my preference.