Well, it depends what you want to accomplish. If you think per city unhealth is too high, change that value from 4 to 3, 2, 1 or zero. If you think per population unhealth should be less than 0.75, change that number. Then save that file back to its original location. (Suggest saving a back-up copy of the original file -- just rename its file extension from .xml to .old, or some such.)