According to the anthropometric proportions of the human head to the body 1/8
So if skin size ~32 pixels(square), then head size D=~3,33 pixels(circle).
The exact hit in an area with such a size is difficult.
The complexity of a shot in the head increases with the distance to the target.
I already wrote that the standard icon in resolution 640*480 looks like shit.
FaqAp: offered me the following idea:
Use left icon for standard headshots (normal hit),
and use the right icon for the ZOOM mode.