The best thing to remember about the vents is that they have one function, and one function only:
To regulate the amount of oxygen that can get to the coals.
The more oxygen that gets to the coals, the hotter the fire.
You can also think of it this way:
The more open the vents, the hotter the fire. The more closed the vents, the cooler the fire.
This is a general statement, of course.
Then, as you start adding more variables, you have to adjust accordingly.
Therefore, on a windy day, you will want the vents more closed - otherwise, more air will be blowing over your coals.
If you are cooking in a very cold environment, you will want the vents open to help the fire offset the cold exterior.
If you are cooking on a hot day, shut the vents down more because the sun will help to heat the cooker - and less fire will be neccessary.
Just remember that the bottom vents control the oxygen.
The top vent should always be open.