It means like don't over use it or don't under use it. Use it for when you need it and when its important rather then use it for something that doesn't matter.
Based off of the information given, the answers should be She is pleased with herself for making Higgins lose his temper, because she knows she hurt him and she only wants the ring for its cash value. This is because she goes automatically to look for the ring, which holds some significance. The way that you can tell she is pleased at making Higgins lose his temper is because she does an almost theatrical like performance that mocks his actions of leaving.
I would paraphrase it being that it is one of the given options.
Application, order, adjustment.
