Refraction, reflection, and diffusion all come into play when we talk about the suns light passing through the air.
You can NEVER get the effects you wish to get (to explain a flat earth) using the refraction of light...
The refraction index of light in vacuum and air are 1 and ~1.000277 at standard temp and pressure.
Try to get your desired effect using Snell's law n1/n2=sinA2/sinA1, where n1,n2 are refraction indexes and n1<n2, A1 is the angle of incidence and A2 is the angle of refraction.
Go ahead, please, let's see if you can get the sun to appear near the horizon, when it's actually above us, using these simple laws...