So the mathematics that NASA use isn't flawed then, The Horizon curves by: sqrt(radius^2 + distance^2)-radius, equivalent to distance^2/R*2. At 100 km, it the horizon descends 784 metres. So where I live I can stand on a hill which 130 metres high look roughly due south and see a mountain range 35km away and behind that see the top of a mountain which is 800 metres high ans 100km away which means by the curvature calculations that NASA and pretty much every geo scientist uses the mountain would have to be 1584 metres high for me to see it...