This description of the Horizontal Gaze Nystagmus test posted by California DUI Lawyer.

In the manual, for practical purposes only, an eight-inch square template is used to demonstrate and assist in estimating this angle. When used, this devise must be held up so that the suspect's nose is above the diagonal line. One edge of the template is centered on the nose and perpendicular to or at right angles to the face. The person is told to follow the stimulus until they are looking down the 45-degree diagonal. This template is only used for practice purposes in order to recognize the angle. 

A rule of thumb in estimating the 45-degree angle, is observing the stimulus halfway between the suspect's ear and nose on the side being tested or just outside the shoulder area. 

The estimation of this angle is critical, since studies have shown that as the alcohol increases the angle will decrease. This estimation for a specific BAC is not sanctioned by NHTSA or in the manual. This test should not be used to estimate a specific amount of alcohol in the bloodstream. In order to properly score this part of the test, the officer must move the object to the 45-degree angle of gaze, taking about four seconds. As the eye follows the object, jerking may be seen. If nystagmus is observed, the stimulus is stopped. The officer must make an observe the nystagmus to have continued. If it continues, then the officer must make sure that there is still white showing in the corner of the eye. 

The angle is then estimated. If it is prior to forty-five degrees the point is scored. If the nystagmus does not continue, the object must continue to be moved until the jerking is observed again and continues or until the 45-degree angle is reached. If no white of the eye is showing, the eye has either been taken too far to the right, which would be maximum deviation that is being evaluated, or the person has unusual eyes that will not deviate very far to the side. The criterion of onset before 45-degrees only can be used if some white can be seen at the outside of the eye. Too often the officer incorrectly estimates the angle or scores this with no white showing in the corner of the eye or both.