Mac laptops these days are built with backlights strong enough to compete with daylight. In typical working conditions, 100% is way overkill.

For max long term comfort, your display should be no brighter than the ambient light levels. Otherwise--if the display is way brighter--your pupils will have to contract when you look at the screen, and dilate when you look away. Your pupils are controlled by muscles and like any muscles, they can get tired.

I have a 2012 MacBook Air and I rarely have the brightness over 50% indoors. And the new MacBook Pros have way brighter screens.

Sitting in my living room right now, my iPhone is at about 15% brightness.