You're using circular logic to prove your point. Your "experiment" only makes sense if you know the world is flat, which you are trying to prove. The argument is a fallacy by the very definition of an argument, because your answer is also a variable in the equation.
Therefore, you cannot argue that the world is flat with your statement.
Being a Canadian, there's one thing I've learned about conservative Americans like yourself: you like your guns, tall buildings, and fast cars. I can provide strong indications of a round earth using these things.
For example, the CN tower in Canada is the world's largest freestanding structure. It's so tall in fact, that they had to plant lasers at the base of the tower when they were building it. Why, you ask? Well, due to the Coriolis effect, the farther away from the surface of the Earth you are, the faster you're rotating. In effect, if the construction workers built straight up without any references to the ground, the structure would corkscrew and be impossible to build totally straight. Ergo, the construction of tall buildings takes into account a round (and spinning) earth.
Now, guns. If you know any sniper specialists in the Army, they will back this up. When a sniper has to make a shot at extreme distances (over a mile), they cannot simply shoot straight at their target, due to wind and the arc of the bullet, obviously. However, they also need to take into account where on the Earth they are, so they can account for the rotation of the earth. If they do not, they cannot properly hit their target. Ergo, a round (and spinning) earth is a sniper's enemy, but one he has to contend with.
Cars are a little trickier, so I will use commercial airplanes. The proposed shape of the flat earth is like a dollar coin, with all the continents up top. Say a pilot wanted to get from Sydney Australia to Washington DC. In a flat earth, the most direct route will be roughly across the North Pole, but in reality they travel over the Pacific Ocean, which in a flat earth would equate to much more time and fuel wasted. I've seen this argument refuted in other posts as planes stopping at many cities along the way, but that's inapplicable here as there is nowhere to stop over the Pacific. And since commercial airliners like saving fuel, and money, they take the most direct route. So, pilots take a route that highly indicates a round earth, being the most efficient path.
One final morsel of food for thought is time zones. If you look on a map of the time zones, you will see they stretch from pole to pole. On a flat earth, they would need to all begin at true North and radiate outward like bicycle spokes. This analysis does not match the actual arrangement of the time zones, as then Australia and Africa would be one huge time zone, instead of several moderately sized ones, which they actually are.
I understand that a key tenet of Flat Earthers is a worldwide conspiracy to hide the true nature of the earth, in which case my argument, and by extension all others, are meaningless. In that case, do not quote vectors and angles and gradients without actually understanding the scientific method.