I have this in my notes, but unfortunately the link no longer works.
"You can use the standard deviation to look at your overall blood sugar control, or to check on specific times of day such as before breakfast. Just use the rule of thumb developed by Dr. Irl Hirsch, an endocrinologist at the University of Washington in Seattle. His guideline is that the standard deviation multiplied by two should be less than the average blood glucose number."
https://www.bd.com/us/diabetes/page.aspx?cat=7001&id=14131 "
Then there is this old T1 thread
http://www.diabetes.co.uk/forum/threads/standard-deviation.40339/
and if you Google there are quite a lot of articles.