If you are doing protein expression, you don't need to measure OD600. I always estimate by eye. That is because I never follow induction at OD600 = 0.6 rule. This rule is bullshit for a lot of, if not most, proteins. So far, I have worked on more than 100 proteins (and their variants). None of them require induction at OD600 = 0.6. Normally I estimate the OD600 at 0.6~0.8. I reduce the temperature to 16~20 degree for an hour. Add 2 gram/liter of glucose and 1 mM IPTG. Don't worry too much if you find your culture is pretty cloudy due to whatever reasons. Chances are, you will have higher yield than otherwise. However, I will be more cautious when I culture M9, especially in D2O.