A method for determining a transformation between frames of reference for two inertial measurement units (IMUs) is disclosed. The transformation includes biases associated with the measurements of each of the IMUs and the extrinsic details can include relative differences in the position and orientation of the IMUs. The IMUs may be mechanically coupled to a frame so that their movements are related. The IMUs can be coupled to a pair of displays in a head-mounted device (e.g., smart glasses) so that a display spacing between for a pair of stereoscopic displays can be accurately determined and adjusted to match the eyes of the user.