As in, how do I make it so that "Minor Dehydration" kicks in at 300 points instead of 200?

Changing the threshold value under Gameplay -> Dehydration Levels doesn't seem to work.

In my limited opinion, I would say if you can't find a variable in Gameplay -> Settings... that gives the desired effect, then this feature is likely hardcoded, i.e. is likely something that you cannot change by simple use of the GECK.