You have to think of the total distance, don't divide it up to 2 different distances. If you do divide it, you will get the wrong answer.

Incorrect. You *must* divide the total distance into two different distances to get the correct answer. First there is the distance from the observer to the horizon, then there is the distance from the horizon to the house.

Marrrrkjo. That how to get the wrong answer.

That is why the drop over 3 miles is 6' = 72". Correct

Not a drop of 1 mile(8") +(8") + (8") = 24" Incorrect

That is not the method that is being put forth. You must break up the distance in to two segments:

1st-The distance from the observer to the horizon. There is no curve obstructing the object.

2nd-The distance from the horizon to the object. This is the only length of curve that is obscuring the object.