How is a hba1c percentile determined?

Directly: Hba1c is a direct measure of the % of red blood cells that are "glycated", meaning they have unusual carbohydrates stuck to them. Whatever proportion have the sugar on them, that's the %hba1c. Patients with higher average sugars stick more to their red blood cells. It's a wonderfully simple, fortunately valid, concept.

See below: It measures percent of normal hemoglobin molecules in red cells (that carries oxygen), that have glucose chemically attached to the molecule.