I've never once broken my macronutrients down into percentages, like 40/30/30. I just don't think in percentages. I bet most people don't.
Instead, I think of calories and macronutrients in terms of money and budgeting. If you've always been confused by all the percentages, give this simple method a shot.
Here's how it works: First, decide how much "money" you have. Let's say you're dieting hard and have decided that you have 1,500 calories to "spend" on macronutrients (protein, fat, carbs). Well, you know you need at least a gram of protein per pound of body weight, so spend that first:
200g protein = 800 calories (1g protein = 4 cals)
Now let's say that you want to keep your carbs at 100g a day, a common dieting strategy:
100g carbs = 400 calories (1g carbs = 4 cals)
That's gives you 300 calories left to spend on healthy sources of dietary fat: Flameout, milled flax seed, olive oil, etc.
I like this more concrete method because it teaches you to make good food choices. For example, after you spend 50 grams of carbs on a post-workout drink, you'll only have 50 left. Are you going to blow them all on half a cup of a pretend health food, like some soy-laden cereal? Probably not. Instead, you'll discover that you can eat a whole lot of green vegetables and get really full for very little "money."
This strategy is self-teaching, self-correcting, and easy to wrap your head around. Try it!