I personally never had a problem playing with the bar to get a result i wanted. Some maps are inherently dark and need a higher brightness, some are too bright and need lower brightness. I play with the brightness bar frequently especially since i have lighting hitting the screen at different times of the day that may change how bright i want my game to be.
However, having a numbered system from 0 to 100% would be far better than a slider, and probably not too hard to do. I support this change +1
