I guess it's more game experience than calculation? I only know that in general the second and third cities were founded nearly at the same time (1st from gold and 2nd built), and theaters are easier to build in earlier cities for they are at full price. In fact, globalization style is very different from 3-theater style. I don't have many experience, but I've seen some videos. If you do not want an early globalization, maybe 3 is a balance number, for getting writers stably, reaching Democracy early, and not hurting your expand/economy/science too much at the same time? After all, you always need campus for every city for SV.