This is a secret hiding in plain view. Most people don't realize this until they're told about it, so if you haven't realized it on your own, don't worry. I didn't either. When you dial in any of the three parts of the Exposure Triangle, the numbers go up or down, depending on which way you spin the dial, each click increasing or decreasing the number by a different amount each time, seemingly with no pattern to it, other than if you dial it up and down, it'll go back to the same number. Each click is a third of a stop. That means that every three clicks is a full stop of light. No matter which part of the triangle you're adjusting, three clicks of the wheel will give you either one more or one less stop of light. Three clicks up doubles the amount of light in your exposure. Three clicks down gives you half the amount of light. This one simple fact will take getting a good exposure from pure guess work to ... well ... educated guess work. Admittedly, you may still have to play a bit to get the exposure you're looking for, but knowing that three clicks is a stop will allow you to know roughly how far to go up or down. If your image is unreasonably dark, try going up two stops. That's six clicks on the wheel. Easy. If your image is completely blown out, try something drastic. Go down five stops. That's fifteen clicks. Not quite as easy, but if you go in groups of three, it's only five times. Instead of worrying about absolute numbers, you can begin to look at exposure adjustments in terms of stops of light. As discussed in the previous section, this is a much more useful metric because it's relative to the lighting conditions you're shooting in.
For example, let's take a scene where you started with a shutter speed of 1/600. It's really dark and you want to go up two stops. Instead of breaking out a calculator to figure out that you want a shutter speed of 1/150, simply click the wheel down six times and you'll discover that the camera is now set to 1/150. You just saved yourself a whole lot of mental arithmetic. You're out here to take pictures after all, not do math!