Spent a bit of it trying to figure out the best breakpoint for an alternate timeline where, for reasons never really explained, nation-states above a certain population level tend to break in half. I was using the USA for my target number — I wanted to get about five countries out of it — but that leaves a paradoxical amount of too many countries being affected, and not enough. Turns out that using a number of about 70,000,000 million leaves Europe (outside of Russia) pretty much intact, and gives India/China both just not enough countries that I’d have to do something more than make up lines on a map.
So, yeah, I’ve been playing with spreadsheets. Ah, the glamorous life of the amateur gaming writer.
Don’t worry about making a singular number the breakpoint. It’s the idea of population driven division that’s important. Mountainous or intemperate areas will split at a lower size than more arable and coastal areas. So have a range during which division can occur…a split that grows likely the closer it approaches the upper limit.