A method for compensating for warpage or other variations in a cross arm on a pole adopted for the mounting of lighting fixtures at predetermined aiming angles relative to a target space on an adjustable mounting structure comprises designing an angular orientation of the fixture relative to a cross arm based on pre-determined assumptions of the geometry of the cross arm; after installation of the lighting fixture on the cross arm, checking for any variation from the assumed geometry; compensating for any variation in actual geometry when finally aiming the fixture to its target location.