The only things I have assumed were things that were already part of the model.
- Viewing distance is finite, whether we want to describe it as due to perspective or atmoplaner density. We cannot see forever across the earth. We can only see a very limited distance.
- The sun is a projection upon the atmoplane
Combine those two, and the sun can come from an Eastwardly direction as a logical consequence.
If interested futher, there are dozens if videos on YouTube by the wider Flat Earth community who came up with the concept a projected sun on their own, show footage of various sun anomalies, some videos better than others, &.