I think an easy way to look at it is "If I start with 2 grassland gems in the capital BFC, I can easily expand to 4 or 5 cities (monarch - adjust for difficulty) without totally tanking my economy JUST on my capital's
income".
From that statement, you can derive how much commerce is needed to expand
x number of cities.
For example: If I can grow vertically early (like with Charism leader), and I have riverside tiles in my first expansion city, I can plant 6 cottages there while still using the capital for whipping settlers/workers. 6x2=12+2(trade & city center) = 14. That's as much as 2 gems mines and then some. This city can fuel expansion of another 2 - 4 cities depending on distance/difficulty.
Although Charism and/or having lux resource in BFC are niche situations, the thought process behind expansion is the same. How much
do I have available to support my expansion? What was the effect on the science slider when I planted my second city? How much
do I need to support my expansion needs? Where will I get the
to do so? Since commerce is the only source of
until reaching Currency, it isn't so advanced to answer these questions. The trick is to ask them from the very start of the game.
Thanks to Writing being a fairly early tech (and relatively cheap for what it unlocks), science is able to be created while, at the same time, lowering the slider. Balancing for military/barb defense, I'm likely to whip libraries in most of my initial cities. Running scientists will get you the
you would normally seek from the slider which frees the slider up to be lowered (funding expansion) without totally killing your research.