Disclosed is a method for determining a subject's reference head posture, the method including: a) obtaining a movement or a position of at least one eyelid of the subject while he/she moves his/her head up and down starting from an initial head posture wherein he/she directs his/her gaze towards a predetermined direction, the subject keeping his/her gaze directed towards the predetermined direction during the motion of his/her head; and b) determining the reference head posture as a function of a movement or a position of at least one eyelid of the subject during the motion of his/her head at step a). Further disclosed is a method for measuring a distinctive height associated with a frame of a pair of spectacles worn by the subject, as well as a method for mounting and for verifying the mounting of an ophthalmic lens in a frame of a pair of spectacles.