There is a special tool used to measure belt deflection/tightness. The alternator belt on that car is prone to be noisy. It needs to be pretty tight to elimate this condition.
how tight is tight? I did not hear any noise when the mechanic who did the oil change told me the belt was loose. He twisted it about half turn and said it should not be like that. He tightened it up all the way up and now it has not deflection at all. I did my research and found information that Lexus drive belt tension is 115 +/20 lbf. To me visually when I touch it feels like 0 lbf. I read if the belt is too tight it actually can do damage to the pulleys and alternator bearing.