Edit: now realizing that this is exactly what you were saying...

I was thinking more about this and realized that once you decide the first segment, the rest are deterministic. Because the adjacent segments cannot be larger or smaller than containing 10% of the population.

Further, once you define the east or west edge of the first segment, the other edge is also deterministic.

In theory you could calculate every possible arrangement of zones to a certain accuracy, such as to every degree of 360. There's only as many possible combinations as there are starting positions for the first longitude you pick.