How high do you need to be to see Los Angeles from Washington DC?
The website you provide a link for gives the horizon distance as:

$d$ $\approx$ $1.23\sqrt{h}$

where d is in miles and h is height above ground level in feet.

Rearranging the equation

$h$ $\approx$ $(d/1.23)^2$

For a horizon distance of 2300 miles, the height would have to be 3,496,596 feet, or 662.2 miles.

For a flat Earth, it depends on the height of any obstacles between Washington DC &amp; LA and the distance the obstacle is from either LA or Washington. If there are no obstacles, no height is required as light from LA can travel across the flat plane to Washington.

If you want to account for impedance provided by the Rocky Mountains, use Google Earth &amp; draw a line between LA &amp; Washington. A high point of around 9600 feet occurs some 780 miles from LA.

By using similar triangles such a height at Washington would be 28,308 feet.