Earth curvature refraction for dummies

I keep being presented with 'earth curvature experiment' videos recently, by flat/concave earth advocates. It seems to be their favorite "evidence" that Earth is not spherical. Debunking this gets into math which I stink at, regarding refraction. I would like to be sure I have considered all the factors. And I keep looking for a simple rule of thumb, 'for each mile, the line of sight drops this amount, due to refraction' value.

So for example, this '20 mile laser' experiment - https://www.youtube.com/watch?v=O8auPtlRTyM proposes to defy Earth curvature. It is a sloppy experiment, but assuming they are not cheating, and their horizontal line of sight is 65 feet above sea level as indicated, does it make scientific sense to be able to see the laser from 20 miles, when the geographic distance to sea level is 10 miles? And how to explain why it does, in simplest terms?

I am also wondering if the laser could be "skipping" over the surface, kinda like a skipping rock, but in a few, long arcs. So the laser grazes the water near the sea level peak say around 10 miles, then is reflected back upward, and arcs over the 'hump' due to refraction. Thus it covers even more distance than if it were traveling over level ground that does not reflect like water.

Sites explaining refraction calculation are too mathy for me. And I seem to be reading conflicting rules and I am getting different end results. I understand there are several variables, but this video indicates that surveyers use a standard 7% rule- https://www.youtube.com/watch?v=4LPcNxYwhQo But that does not seem to account for the 20 mile laser example. And I understand that the closer to the water, and the greater the distance, the more the laser will refract, or 'super refract.'

WikiP entry on "horizon- effects of refraction- says something about a 4/3 ratio, and 15% beyond geometrical horizon. And 'standard' atmospheric refraction is 8%, although that is not 'super' refraction as I assume would apply to the laser example. But this does not match the 7% rule, and I dont get how all the values are used to get a definitive refraction value.

I see a simple 'Distance to horizon calculator' (this forum wont allow me to post the link, will try to post in another comment) but does not figure refraction.

And another one is suppose to calculate refraction 'lapse rate' , but no matter what I enter, the lapse rate always comes out "0." Doesnt seem to work.

Any clarification is much appreciated.