Lock the Index Mirror when the Index Bar is situated about half way along the arc and see whether the reflected image of the arc is in line with the arc itself.
Tshixa isipili sesalathisi xa umgca wesalathisi ubekwe malunga necala lomgama ngakwisazinge kwaye ujonge ukuba ngaba imbonakalo okanye umfanekiso obonakalisiweyo kwesazinge ukuba ukuye okanye uhambelana nesazinge ngokwaso.