This is another A1C/blood glucose chart that you may see posted in some diabetes groups and other sources. It is based on a study done in 1993 and written about in 2002.

A1C Chart & Calculator DCCT
DCCT formula seems to work best in people with high blood sugars. Because this formula derived from such group.
Reference: Diabetes Control and Complications Trial or DCCT, New England Journal of Medicine 1993; 329:977-986. “Defining the Relationship between Plasma Glucose and HbA1c, Analysis of glucose profiles and HbA1c in the Diabetes Control and Complications Trial,” Diabetes Care 25:275-278, 2002.
http://healthy-ojas.com/diabetes/a1c-glucose-chart.html