New Learnings - The difference between managing and Insulin

Each week my dietician does a check in with all her pregnant women to see how they are doing with managing their diabetes. We talk about the numbers of my glucose tests, what they are and what is making them to high or low. These check ins also allow for me to ask any questions I may have and get some advice on diet questions or ways to assist in managing my gestational diabetes. During my last check in I learned that the difference between my readings being good and possibly needing to add insulin to my management plan is 80%. This means that as long as my good readings, Ie. those in the acceptable range, are greater than 80% I am considered to be managing my diabetes well. I wonder thou how this measurement was decided? Now just to see what my doctor says on Monday at my next appointment.