I can't really think of a better way of deciding in what percentage the map should be split.
Also, mirroring the map (which won't happen for now due to technical reasons) does not solve this problem, because the percentage would still have to make sense. Because otherwise one region would be way emptier than the other, and that's unfair for those who happen to be in the region that's packed. That might even force people to go to the other side, just because it's less populated, which renders the whole idea of splitting the map useless.
tl;dr: find me a better way to decide in what percentage to split strategus. But something that can be calculated, not just some gut feeling.