Remember when I said that the earth wasn't as flat as a mirror (being that there are things on the ground that reflect the light in all different directions)? Do you remember that part? If you don't, you can scroll back up and read it. Also, did you notice how flat the earth looks in your pic? I did. In fact, your measurements seem to rely heavily on that right angle.
Then I guess my graph isn't very reliable for that purpose. But to disprove the idea of straight light on a flat earth, we only need to look at the sun rise on a clear morning. It's completely clear, the atmosphere is thick enough to protect our eyes from most of the harmful rays. The sun rises, top first. It starts at 0º and begins to rise higher into the sky.
Let's calculate the angle of the sun on a morning according to the flat earth. The perimeter of the equator is about 40.000km around, am I wrong? So the sun would be 1/4 of this distance around the circle: 10.000km away, we'll just consider this the horizontal distance despite the curve of the equator. Let's see what angle of the sun would be at sunrise:
hDist=10.000: Horizontal distance of the sun
sunHeight=32.000: Vertical distance of the sun
totalDist: Shortest distance between earth and sun
Alpha: Angle of the sun compared to the ground, as seen from the observer.
totalDist = sqrt(sunHeight*sunHeight+hDist*hDist) -->
totalDist = 33526.
sin(Alpha)*totalDist = sunHeight -->
sin(Alpha) = sunHeight/totalDist -->
Alpha = arcsin(sunHeight/totalDist) -->
Alpha = 72.64º
This means the morning sun starts way up high in the sky. It's not even close enough to the earth in order to make the illusion of being behind the horizon.