I normally keep my screen brightness at almost the absolute minimum since that control only affects minimum brightness. Generally, this setting is what's used when you're driving at night. During the day, a photocell adjusts the brightness upward as much as necessary.

Interestingly, I just had my screen assembly replaced because that photocell failed. The failure meant that I had to start adjusting that brightness control to full during the day and then back down to minimum at night. If you're doing this, you've got a bad photocell too. When everythings operating normally, you should never have to touch the brightness control on a MKIV once you've got it adjusted to a comfortable nighttime setting.