For an accurate measurement, it is essential that both ears (and only the tip of the nose) are seen.
place the included earspreaders behind the ears
pull the hat on tight so that the ears stick out
make sure the ears are not inside or covered by the hat
left ear missing?
move your camera slightly more to the left
right ear missing?
move your camera slightly more to the right
trouble with both ears?
try an angle slightly further away - but make sure that not more of the face is visible