The bottom of the buildings isn't the strongest point this video makes. She is making the point that the entire building should be below the horizon. With enough curve, and by the way the formula is 8 inches per mile squared, so 8x(M2), that means the entire damn building should be below water... We're talking exponential curve, at just a few miles it comes out to thousands of feet and this is taken from the diameter of the earth using curve trigonometry.

Now, with that being said - the bases of the building isn't as important as the entire building should be under the horizon and isn't...