It's just your base output #, pre-modifiers, which is the sum of everything your cities give you.
Golden ages do not count.
Nothing from CS counts.
Honor opener doesn't count.
Anything that affects your cities DOES count, which is why the world fair counts. It doubles YOUR culture output (meaning culture from your cities; not the culture output of anything else; golden age is treated as an alien entity; so are trade routes; so are city states; so are bonuses from policies that are not tied to your city's base output).
It's just your base output #, pre-modifiers, which is the sum of everything your cities give you.
Golden ages do not count.
Nothing from CS counts.
Honor opener doesn't count.
Anything that affects your cities DOES count, which is why the world fair counts. It doubles YOUR culture output (meaning culture from your cities; not the culture output of anything else; golden age is treated as an alien entity; so are trade routes; so are city states; so are bonuses from policies that are not tied to your city's base output).
Indeed it seems to count all culture past 8 turns as it changed drasticly near the end of a golden age + worlds fair 100%.
Tourism from Musicians works differently however as it only seems to multiply when born and remain static.
This is not correct for Great Writers. It is correct for Great Scientists (neither the Rationalism opener nor Scholasticism, nor beakers from RAs of prior GS bulbs, is taken into account in determining the payoff from bulbing a GS).
For Great Writers, the political treatise pay-off is simply the sum of all of your culture for the last 8 turns, from all sources. Below is a chart that shows this for a current game (the totals differ by 4 because the UI rounds decimal amounts up and down for display purposes, but uses actual decimal amounts in determining the payoff from bulbing):
Turn|Culture from cities|Culture from excess happiness|Culture from CSs|Culture from Golden Age|Total culture
186|197|32|156|77|462
187|203|32|156|78|469
188|306|33|156|99|594
189|307|32|156|99|594
190|305|32|156|98|591
191|372|33|156|112|673
192|372|33|156|112|673
193|378|33|156|113|680
Total|2440|260|1248|788|4736
GW bulb yield on turn 193: 4732
In general, GS I plant until Plastics, then I bulb. GW I I make writing until I hit Archeology, then I bulb, since my explorers will start findin artifacts, and the culture from Political Treatise will skyrocket