The circumference of the skull measured at the temples is used for the size of the normal hats in soft and flexible materials.
All that is needed is to know if the extension of the headband can easily surround the head of the wearer; the flexibility of the material does the rest, and makes comfortable use.
A helmet, however, is neither flexible nor adaptable, it is rigid and the shape of the human skull is evidently very different from a sphere.
The section of the skull and of the helmet are similar to an ellipse, whose basic parameters are the two major and minor diameters (A and B).
Here’s how to measure them: