brix measurment

Hi I wonder if any one can help me I am new to this and would like some advice about measuring the sugar content in foliage of plants. I have found a little bit of information in the wonderful Indoor marijuana horticulture in regard to brix meters but this has only wet my appetite, I want to know more, I understand the principle of how they work but I would like to know more regarding the interpretation of the results for instance if you have a plant that is unhealthy and you are getting a low brix measurement how do you interpret the reading do you just ad the nutrients the plant should be on at that stage of growth or are there ways to tell what the deficiencies are , are there ways to tell if your plant are locked, or is this all trial and error. cheers