The frontality angle describes how "frontal" the pigtails are. It is defined as the angle between the line connecting the pigtail origin to a vertical axis running through the head centre and the vertical plain dividing the head into left and right halves.
For example: a regular ponytail (which may be thought of as a set of pigtails at identical positions), would be at a frontality angle of zero, being located at the back of the head. Pigtails originating above the ears would be at 90 degree frontality.
Pigtail frontality angles range from 15 to about 100 degrees.

Usually, the pigtails originate somewhere above the ears, so frontality values are normally around 90° (fig. b). At frontality values of less than 90°, the pigtail origins are hidden from view when facing the girl wearing the pigtails (fig. c).

90° frontality
fig. b: 90° frontality
75° frontality
fig. c: 75° frontality