[magnetaress]

If the earth’s curve is at 8 inches per square mile, how can one see objects over 70 miles away?
The earth curves away at roughly 8 inches per mile squared over short distances. (Not square mile)

The formula to determine how far you can see depends entirely on your viewing height.

At six feet off the ground you can see the ground for 3 miles around you and of course anything further away that's tall enough to not be obscured by the curvature.

At 3250 feet the horizon is 70 miles so if you’re on a hill, you could see all the way to the ground 70 miles away.

One mistake some people (ahem flat earthers ahem) make is they will calculate the distance to the horizon viewed from 0 inches off the ground when they are actually much higher. The forget to add their height, plus the height of the sand dune they’re on.

Then they mistake seeing anything with seeing everything.

They will look at the Chicago skyline from the dunes park on Lake Michigan and say it shouldn't be possible because there is 1700 feet of curvature without realizing they are only seeing the tops of the tallest buildings.

The other factor is refraction. Light travels at a different speed in dry, warm air as it does in moist, cool air and if you’re looking across a body of water, those conditions exist. It's only a percent or two but it allows you to see more that you would just based on the calculation.

The formula isn't completely accurate either.

Do the calculation between the furthest on earth, 12,450 miles apart.

12,450 squared is 155,002,500, times 0,667 feet (8 inches) and you get 103,386,667.5

Divide that by 5280 and the answer is 19,580.8 miles of drop over 12,450 miles.

Obviously that can't be correct so the formula an increasingly inaccurate result the longer the distance is.

The circumference of the Earth is 24,900 miles, divided by the degrees in a circle and we get one degree of curvature per 69.1 miles.