Depends on the distance between their hometowns (assuming they lived elsewhere before the job) and the population of the areas. Then it is 1/X with x being the population. You could also do it as 1/J with J being Japan's population at the time.
Every time this trope pops up it feels so insecure. Like the writer doesn't trust the love story to be interesting on its own merits, so oop let's make them fated to marry from when they were kids. Not everyone has to be a soulmate.