What is The Best Method to Use to Measure Earth Curvature?

Like I said about proportions applies here too. I can't see the next continent or islands across the sea. When the sun travels it eventually goes out of view. Even the moon does that most of the time. A major proponent of the flat earth theory said that it is a liquid and it solidifies every night as it emerges from the sea. He shows photographs of it with wavy hazy appearance on the horizon of the flat water to "prove" his fact.

I was top of mine, also. Does that make you feel better?

No. It’s disturbing how the quality of education and rigour of examination varies so widely.

The "8 inches per mile/squared" is the easiest.

Also if you can go somewhere, or even find many scenes on TV and/or Youtube, etc. where you can see a long horizon in the background, especially if it's a water horizon, you can see very far from side to side that there is not curve at all.

Some have made the mistake of calculating 8 inches per mile, but that would make the drop of the horizon in the distance to go down at a diagonal angle. Because the earth is 'supposed' to be round, the mile/squared part is essential. That means that it should drop slightly at first, then drop dramatically - as it is 'supposed' to be a sphere.

8 inches x miles (distance) squared

The 1st mile it should drop 8 inches.
The 2nd should drop 32 inches.
The 3rd should drop 72 inches, or 6 feet.

Find the farthest place you can go and still see the tallest city buildings near you and then calculate the distance. Using the formula above, check and see if the correct number of feet are missing from view from the bottoms of the buildings. Sometimes the geography of the land between can obfuscate the bottom of the buildings, but if you calculate it all properly, you will see that they are not hidden from view as they should be, nor are they leaning away at all.