Even better: A spread culture mission will always yield at least 1It has plot culture but not city culture. The spread culture mission gives 5% of the total existing city culture, so that's a minimum of 20(ours and his combined) just to spread 1
.
Does not have to include any of our city culture though. (tested)


Proof:
CvPlayer.cpp said:// Insert Culture into City
if (NULL != pPlot && pPlot->getCulture(getID()) > 0)
{
int iCultureAmount = kMission.getCityInsertCultureAmountFactor() * pCity->countTotalCultureTimes100();
iCultureAmount /= 10000;
iCultureAmount = std::max(1, iCultureAmount);
iMissionCost = iBaseMissionCost + (kMission.getCityInsertCultureCostFactor() * iCultureAmount) / 100;
}