The High Chaos thresholds are for overall chaos. There is no per mission chaos.

For example:
 - If you kill 8 guards in prison, you'll have 12 chaos the end of mission screen will say High Chaos.
 - If you then kill 6 civilians in Campbell (as well as Campbell himself, since sparing targets takes away 20 chaos) you'll get 36 chaos for a total of 48 chaos, enough for high chaos in that mission.
 - If instead you didn't kill anyone in prison, and go into Campbell with 0 chaos, but otherwise do the mission exactly the same you'll have 36 chaos at the end, and the end of mission screen will say low chaos.