Trade with worst enemy:
The thing is working different than other negatives.
It doesn't matter how often you trade with worst enemies, it's the value relative to the turns you know the one giving you negatives.
Edit: I just see a misunderstanding in FiveAces post:
There is only a max of 1 worst enemy for a civ at any given time (it can change).
It's the one at annoyed or below with the lowest attitudeVal (the numeric value with all the hidden modifiers one cannot really know
).
You have an allowance of 10*HasMetCounter before you get 1 negative. Gifts (as e.g. giving in to demands
) count full, trades with 1/3 the value of your side of the trade.
Open borders have nearly no value (total number of cities of both partners), health or happiness resources count 100. It's techs which amount for a lot (1.5 - to nearly 2 times the beaker value depending on how many civs know the tech).
The mechanism for getting fair trade +points is similar. Notable there is that gifts count the same as excess trade value and the divider is 5*HasMetCounter.