It looks like AI plot when you are at most [80-130 dependent on the AI] percent of their power. The "usual suspects", i.e. izzy, monty, genghis, and such tend to be the ones who will plot even if you are at 120 or 130% of their power, whereas the more creampuff AI require you to be weaker. Willem's threshold is 100% for what it's worth, but at 3.0 times you are all good by quite a bit! Of note is that those numbers are for a "total war", and also for when the AI is located nearby (land target perhaps?? source doesn't specify). If the AI is further away, they require much more strength to plot (the range changes to 30-100%). The AI can also plot "limited wars", following similar but not identical rules, as well as bribes and daggers which don't necessarily require plotting at all and thus might sidestep the usual numbers a little.
Note on BUG: you can set the ranges where power ratios will show green/white/red, but I think the default is below 0.8 is red and above 1.2 is green, which would make quite a bit of sense given the war plot ranges.
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