Well I think it's a given you want your 2nd city ASAP, on any difficulty level. The question is how fast to expand beyond your first 2-4 cities.
60% is a good baseline. You will want to stray from the baseline at times, but do it temporarily with a plan on how to get back to it.
When considering a new city, ask yourself: how will I pay for it? Obviously if you are running a surplus at 80% you can afford a new city. But even if you have to drop below 50% for a bit while you bring the new one "online", it may very well be worth it in the long run.
This is where good planning comes in. It's ok to strain your economy for a city that you can see will ultimately be an economic powerhouse (lots of food, a gold mine, etc.) If, on the other hand, you foresee a new city being a production site, you have to make sure that your other cities can generate the cash to support it.
So as a rule of thumb, while running at 60%, I will hinder my science if I see a juicy city spot that I know will pay for itself before long. Otherwise, I will only expand if my economy is producing surplus.